Improvement #7: Add GLX_OML_sync_control VSync support

- 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().

Makefile

@@ -44,6 +44,7 @@ endif
 # ==== OpenGL VSync ====
 ifeq "$(NO_VSYNC_OPENGL)" ""
   CFG += -DCONFIG_VSYNC_OPENGL
+  # -lGL must precede some other libraries, or it segfaults on FreeBSD (#74)
   LIBS := -lGL $(LIBS)
 endif