Original commit message from CVS:
Patch from Gernot Ziegler <gz@lysator.liu.se> rewriting the
GL sink plugin. (Bug #147302)
* configure.ac: Test for OpenGL
* sys/Makefile.am: Use test for OpenGL
* sys/glsink/Makefile.am:
* sys/glsink/glimagesink.c: rewrite
* sys/glsink/glimagesink.h: rewrite