Imp: Fix GL_TEXTURE_RECTANGLE & Enhance --glx-copy-from-front

- Fix GL_TEXTURE_RECTANGLE support. Thanks to amonakov for guides.
  (#107)

- Enhance --glx-copy-from-front to improve performance and make it work
  with --glx-swap-method, copied from kwin patch. Thanks to bwat47 for
  info. (#107)

- Add texture2Doffset() support in blur GLSL shader. Thanks to amonakov
  for advice. No visible benefit here, though. (#107)

- Only limited tests are done and I'm super sleepy. Bugs expected
Misc: Add DEBUG_GLX_PAINTREG

- GLX: Add DEBUG_GLX_PAINTREG, for debugging painting region issues, for
  p4ddy's problem.
Bug fix: Compilation failure with NO_LIBCONFIG / NO_C2

- Fix compilation failure with NO_LIBCONFIG or NO_C2. Thanks to
  Spaulding for reporting.
Misc: Add DEBUG_GLX_MARK & Misc

- GLX backend: Add DEBUG_GLX_MARK, to add GL marks around functions with
  glStringMarkerGREMEDY(), and mark frame termination with
  glFrameTerminatorGREMEDY().

- Print output of `compton -h` to stdout. (#110)

- GLX backend: Strip out elements with factor 0 in GLSL blur code.
  Thanks to jrfonseca for guides. (#107)
Merge branch 'richardgv-dev'