- 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().