Bug fix: GLX: Fix --inactive-dim & fix color inversion

- GLX backend: Fix broken --inactive-dim.

- GLX backend: Fix bugs when inverting colors of windows. Thanks to
  madsy and neure for help.

- GLX backend: Lift `glx_no_stencil` restriction from glx_init_blur().
  It still probably won't work, but the user can try.

- XRender backend: Use XRenderFillRectangles() instead of
  XRenderComposite() to do dimming.