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