- Add "vsync-oml" VSync method, using GLX_OML_sync_control. Untested,
because it's not supported by my driver.
- Unredirect ps->reg_win, because DRI wiki says it's related to the
behavior of OpenGL VSync extensions.
- Add glFlush() and glXWaitX() calls, in hope they are slightly helpful
for VSync.
- Change a few functions to make error handling more graceful. Make some
errors fatal. Code clean-up.
- Add unused function make_text_prop().