Feature: Add XRender-GLX hybird backend

- Add new backend "xr_glx_hybird", which uses X Render for all
  compositing but GLX on the last step of rendering to screen.  This
  makes GLX-backend-specific VSync methods usable while may avoid
  certain bugs with GLX backend. The idea comes from ali1234.
  Experimental.

- GLX backend: Stop using or rendering to depth buffer.

- Use glFinish() instead of glFlush() before VSync. It probably uses
  more CPU but could be more reliable than glFlush().