Feature: OpenGL backend

- Add experimental OpenGL backend (--opengl). --blur-background is
  currently not possible with this backend, because I'm still trying to
  find a proper way to do blur with OpenGL. Flipping backend on-the-fly
  is really hard, so it isn't supported right now. No configuration file
  option exists to enable this, because it isn't stable enough.

- Add `opengl-swc` VSync method that uses SGI_swap_control to control
  buffer swap, with OpenGL backend. (#7)

- Fix a potential read-from-freed-memory issue in paint_all().

- Correctly reattach GLX context after fork.

- Dump error text in error(). Add GLX error code handling.

- Code clean-up.

- Known issues: Region operations take a lot of time in glx_render().
  I'm hesitating about what to do.
Bug fix: GLX backend incompatibility with mesa & others

- Fix a bug that glx_bind_pixmap() doesn't work with mesa drivers.
  Thanks to Janhouse and mkraemer for reporting. (#7)

- Use stencil buffer to attempt to eliminate potential double-paint
  issue in glx_render(). X Fixes doesn't guarantee the rectangles in a
  region do not overlap, and this may cause some regions to be painted
  twice, which would be a problem if we are painting transparent things.
  Now the target window must have a stencil buffer. Compiz uses its own
  region implementation to deal with this, but as a lightweight
  compositor we can't really do the same. It may have a positive or
  negative effort over performance. Callgrind result indicates basically
  no change in performance, but this may or may not be true.

- Correctly distinguish GL extensions and GLX extensions. Sorry. :-D

- Handle screen size. Thanks to tsmithe for reporting. (#7)

- Rename OpenGL backend to GLX backend, because, we might have a EGL
  backend someday.

- Add configuration file option `backend` to specify backend. Add
  `backend` to D-Bus `opts_get`.

- Add OpenGL shader compilation code, but currently unused.

- Minor adjustments.

- Known issue: Window content doesn't get updated in VirtualBox,
  probably because its OpenGL implementation requires constant rebinding
  of texture. But that's really slow...

- Known issue: Blur feature is still unimplemented in GLX backend.
Bug fix: GLX backend incompatibility with VirtualBox & others

- GLX backend: Fix a bug that window content does not get updated on
  VirtualBox, by rebinding texture when window content changes. This may
  have a negative effect on performance.

- GLX backend: Add --glx-no-stencil to restore the old clipping method,
  just in case.

- GLX backend: Apply stricter checks on texture-pixmap binding.

- GLX backend: Fix a bug that glx_set_clip() behaves incorrectly when
  None is passed in.

- GLX backend: Use glEnable()/glDisable() to toggle stencil tests, in
  hope to increase performance.

- Move window pixmap/picture fetching to win_paint_win(), in hope to
  increase performance.

- Intersect shadow painting region with its bounding rectangle, in hope
  to increase performance.
Bug fix: GLX: ARGB texture too dark & Jitter when resize & others

- GLX backend: Fix a bug that ARGB windows / shadows are rendered too
  dark. Thanks to derhass in FreeNode/##opengl for help.

- GLX backend: Fix a problem that during window resize the content looks
  jittering, by letting compton fetch pixmap sizes with XGetGeometry()
  instead of relying on window width/height, which could be inaccurate
  during window resize. Negative effect on performance. Thanks to M4he
  for reporting. (#7)

- Add .desktop file. Thanks to quequotion for providing it. (#97)

- Avoid checking presence of window pixmap, because they may not exist
  with very old X Composite implementations.

- Add workaround for a strange window restack issue when compton
  receieves a ConfigureNotify with non-existent new above window.

- Add debugging function hexdump(). Extra sanity checks on various
  places.
Improvement: GLX: Use SCISSOR_TEST instead of STENCIL_TEST when possible

- GLX backend: Use GL_SCISSOR_TEST instead of STENCIL_TEST if there's
  only one rectangle in glx_set_clip(). Profiling with gDebugger shows a
  10% performance improvement.

- Add .desktop installation rules. (#97)
Improvement: --glx-copy-from-front & benchmark mode

- GLX backend: Add --glx-copy-front, which copies all unmodified regions
  from front buffer to back buffer instead of redrawing the whole
  screen. Unfortunately, probably because glCopyPixels() isn't usually
  well-optimized on graphic cards, this option appears very unstable in
  its effect: Over 20% boost in performance when only 1/4 of the screen
  is modified, but 10% decrease when the whole screen is. Thus, not
  enabled by default.

- GLX backend: Add glx_blur_dst(), to prepare for the background blur
  feature. It currently is capable to modify background in the desired
  way, but the core blur shader is absent. glConvolution2D() seemingly
  relies entirely on CPU and has horrifying performance. I've hesitating
  about whether I should use ARB assembly language or GLSL for the
  shader.

- GLX backend: Invert y-axis GL matrix. It's upside-down previously
  because that's what dcompmgr uses. Seemingly a "normal" y-axis is
  required for glCopyPixels() to operate correctly.

- GLX backend: Fix some GLX_TEXTURE_RECTANGLE compatibility issues.
  Still, not actually tested.

- Add benchmark mode (--benchmark & --benchmark-wid).

- Misc changes.
Feature #69: GLX: Blur background

- GLX backend: Add blur background support using a GLSL shader. Only
  tested with nvidia-drivers-313.26. Known to cause quite some decrease
  in performance (~10%?).

- Detach shaders in glx_create_program(). Misc changes.
Improvement: --glx-use-copysubbuffermesa

- GLX backend: Add --glx-use-copysubbuffermesa, to use
  MESA_copy_sub_buffer to do partial screen update. Huge performance
  boost on mesa drivers for partial screen updates, but does not work
  for nvidia-drivers and may break VSync. Automagically overrides
  --glx-copy-from-front.

- Add rect_is_fullscreen() to reuse code. Misc changes.
Bug fix: Incorrect handling when a window is placed on bottom

- Fix a bug that ConfigureNotify placing a window in bottom
  (ConfigureNotify with .new_above == 0) is not correctly handled,
  introduced in 1a88e3d0c5.
Improvement: --blur-background-exclude #98 & MESA_swap_control & others

- Add --blur-background-exclude. (#98)

- Add `opengl-mswc` VSync, which uses MESA_swap_control instead of
  SGI_swap_control. I don't expect it to perform better than
  SGI_swap_control, though.

- Update CMakeLists.txt .

- Add a few targets for D-Bus `win_get`. Misc changes.

- Known issue: Apparently I've forgotten implementing --inactive-dim on
  GLX backend... Silly me.
Bug fix: GLX: Fix --inactive-dim & fix color inversion

- GLX backend: Fix broken --inactive-dim.

- GLX backend: Fix bugs when inverting colors of windows. Thanks to
  madsy and neure for help.

- GLX backend: Lift `glx_no_stencil` restriction from glx_init_blur().
  It still probably won't work, but the user can try.

- XRender backend: Use XRenderFillRectangles() instead of
  XRenderComposite() to do dimming.