Bug fix: GLX backend incompatibility with VirtualBox & others

- GLX backend: Fix a bug that window content does not get updated on
  VirtualBox, by rebinding texture when window content changes. This may
  have a negative effect on performance.

- GLX backend: Add --glx-no-stencil to restore the old clipping method,
  just in case.

- GLX backend: Apply stricter checks on texture-pixmap binding.

- GLX backend: Fix a bug that glx_set_clip() behaves incorrectly when
  None is passed in.

- GLX backend: Use glEnable()/glDisable() to toggle stencil tests, in
  hope to increase performance.

- Move window pixmap/picture fetching to win_paint_win(), in hope to
  increase performance.

- Intersect shadow painting region with its bounding rectangle, in hope
  to increase performance.