Improvement: Separate GLX parts from session_t & Attempt to fix #217

- Separate GLX parts from session_t into glx_session_t.

- Add --rererdir-on-root-change and --glx-reinit-on-root-change, as
  possible solutions for #217. Thanks to jlindgren90 for reporting.
Misc #218: Warn about using --glx-no-rebind-pixmap on intel driver

Warn about using --glx-no-rebind-pixmap on xf86-video-intel. Thanks to
aktau for the info. (#218)
Misc: Fix compilation with NO_VSYNC_OPENGL_GLSL / NO_C2

Fix compilation with NO_VSYNC_OPENGL_GLSL / NO_C2.
Bug fix: Fix X pixmap leakage in shadow_paint

- Fix X pixmap leakage in shadow_paint.

- Add the skeleton of a X resource leakage checker.
Changed "guassian" to gaussian .
Merge pull request #221 from treeman1111/patch-1

Changed "guassian" to gaussian .
Misc: Fix spelling mistakes

Still, "Guassian" -> "Gaussian". (#221)
fix crash caused by free of uninitialized pointer
Merge pull request #223 from maep--/master

fix crash caused by free of uninitialized pointer
Misc: Add --no-name-pixmap

Add --no-name-pixmap to disable the usage of
XCompositeNameWindowPixmap(), for debugging.
Merge branch 'richardgv-dev' of github.com:chjj/compton into richardgv-dev
Misc #204: Add glx_take_screenshot() & others

- Add glx_take_screenshot() for taking a screenshot. With ImageMagick
  the output data could be viewed in this way:

    display -size [SCREEN-WIDTH]x[SCREEN-HEIGHT] -depth 8 -flip
    rgb:[PATH]

  (#204)

- Add D-Bus command `opts_get string:paint_on_overlay_id` to get X
  Composite overlay window ID. (#204)

- Code cleanup.