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.