sys/glsink/: Convert gldownload to BaseTransform. Make glfilterexample visually...

sys/glsink/: Convert gldownload to BaseTransform.  Make glfilterexample visually interesting.  Add support for variou...

Original commit message from CVS:
* sys/glsink/gltestsrc.c:
* sys/glsink/gstglbuffer.c:
* sys/glsink/gstglbuffer.h:
* sys/glsink/gstgldownload.c:
* sys/glsink/gstglfilter.c:
* sys/glsink/gstglfilterexample.c:
* sys/glsink/gstgltestsrc.c:
* sys/glsink/gstglupload.c:
Convert gldownload to BaseTransform.  Make glfilterexample
visually interesting.  Add support for various formats to
downloading.  Fix a few places where we leak GL state to
other elements (bad, but hard to prevent).