sys/glsink/: Switch to using framebuffer_objects instead of GLXPixmaps, because that...

sys/glsink/: Switch to using framebuffer_objects instead of GLXPixmaps, because that's what my driver supports.  Remo...

Original commit message from CVS:
* sys/glsink/Makefile.am:
* sys/glsink/glextensions.c:
* sys/glsink/glextensions.h:
* sys/glsink/glimagesink.c:
* sys/glsink/glimagesink.h:
* sys/glsink/glvideo.c:
* sys/glsink/glvideo.h:
* sys/glsink/gstglbuffer.c:
* sys/glsink/gstglbuffer.h:
* sys/glsink/gstgldownload.c:
* sys/glsink/gstglfilter.c:
* sys/glsink/gstglupload.c:
* sys/glsink/gstopengl.c:
Switch to using framebuffer_objects instead of GLXPixmaps,
because that's what my driver supports.  Remove GLDrawable,
since GstGLDisplay now has a default drawable and context.