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.