Misc: Fix wrong description & DEBUG_GLX_ERR

- Fix description of "opengl" VSync.

- Add DEBUG_GLX_ERR to check for OpenGL errors.

- Update man page.
Improvement: --blur-kern

- Add blur convolution kernel customization, --blur-kern. The format is
  a bit tricky so be sure to read the description in `compton -h`. Not
  much tests received.

- GLX backend: Tolerate missing GLSL uniforms for strangely shaped
  convolution kernel.

- Fix a memory leak that blur-background blacklist is not freed.
Improvement: Enhance --glx-swap-method

- Enhance --glx-swap-method to support longer buffers ages (3-6), and
  automatic buffer age detection via GLX_EXT_buffer_age.
Misc: Fix gcc error

Fix gcc compilation error about "initializer element is not constant".
Improvement: --resize-damage

- Add --resize-damage to enlarge/shrink repaint region by a specific
  number of pixels, used for solving the line corruption issue with
  blur. Thanks to Nuck and jerri in #104 for reporting.

- Fix the memory leak of blur shader string.
Bug fix: Fix --resize-damage

- Fix --resize-damage. I forgot to shrink the painting region back when
  actually copying to destination.

- Include extra pixels around the blur texture to avoid some possible
  small issues, if --resize-damage is positive.

- Known issue: Line artifacts may still appear with --dbe (X Render
  backend) or --glx-swap-method (GLX backend). I doubt if there's way to
  fix this without very inefficient mechanisms.
Bug fix: Fix a BadRegion error with --glx-use-copysubbuffermesa

Just a small and mostly harmless issue. Sorry, didn't test throughly.
Improvement: Add predefined blur kernels

- Add a few predefined blur kernels, requested by jerri in #104.

- Add compton-convgen.py to generate blur kernels.
Misc: Validate wallpaper pixmap & Documentation update

- Split Pixmap validation out to validate_pixmap(). Validate wallpaper
  Pixmap as well.

- Update README.md and man page.
Misc: Documentation update

- Just fix some small issues in README.md .