Misc: Workaround for some missing definitions

- Add workarounds for missing GL_TEXTURE_RECTANGLE and PictOpDifference
  definitions in broken GL headers / old X Composite headers.
Improvement: GLX: Cache region contents & --glx-no-rebind-pixmap

- Cache region contents in is_region_empty(), mostly useful only for GLX
  backend to save one roundtrip to X.

- GLX backend: Add --glx-no-rebind-pixmap, which prevents rebinding of
  GLX texture to pixmap on content change. It doesn't work on some
  drivers, but it saves some CPU on those where it does.

- Wrap XFree() with a new function cxfree() since its man page claims
  NULL pointers are not acceptable (although in fact it does...).

- Use macro to save some code in get_cfg(). Code clean-up.
Bug fix: GLX: Fix dim and blur with --glx-no-stencil

- GLX backend: Fix broken dim and blur with --glx-no-stencil when
  dealing with shaped windows.

- GLX backend: Cache region contents and do a local region intersection
  instead of using XFixesIntersectRegion(). Drastic reduction in CPU
  usage for --glx-no-stencil. Now --glx-no-stencil substantially
  outperforms (~15%) normal mode.

- Use macros to reuse paint-in-region code in opengl.c . Add new type
  "reg_data_t" to store XserverRegion cache.