Bug fix: GLX backend incompatibility with mesa & others

- Fix a bug that glx_bind_pixmap() doesn't work with mesa drivers.
  Thanks to Janhouse and mkraemer for reporting. (#7)

- Use stencil buffer to attempt to eliminate potential double-paint
  issue in glx_render(). X Fixes doesn't guarantee the rectangles in a
  region do not overlap, and this may cause some regions to be painted
  twice, which would be a problem if we are painting transparent things.
  Now the target window must have a stencil buffer. Compiz uses its own
  region implementation to deal with this, but as a lightweight
  compositor we can't really do the same. It may have a positive or
  negative effort over performance. Callgrind result indicates basically
  no change in performance, but this may or may not be true.

- Correctly distinguish GL extensions and GLX extensions. Sorry. :-D

- Handle screen size. Thanks to tsmithe for reporting. (#7)

- Rename OpenGL backend to GLX backend, because, we might have a EGL
  backend someday.

- Add configuration file option `backend` to specify backend. Add
  `backend` to D-Bus `opts_get`.

- Add OpenGL shader compilation code, but currently unused.

- Minor adjustments.

- Known issue: Window content doesn't get updated in VirtualBox,
  probably because its OpenGL implementation requires constant rebinding
  of texture. But that's really slow...

- Known issue: Blur feature is still unimplemented in GLX backend.

compton.sample.conf

@@ -34,6 +34,7 @@ fade-out-step = 0.03;
 # no-fading-openclose = true;
 
 # Other
+backend = "xrender"
 mark-wmwin-focused = true;
 mark-ovredir-focused = true;
 use-ewmh-active-win = false;