Provide bindings and subclass for GstGLBaseSrc
Fixes #219 (closed)
Whipped this up in ~10min, seems trivial enough to safely implement including subclass. gl_start/stop
was copied from gl_base_filter
. fill_gl_memory
uses same semantics as some gl_filter
functions taking GLMemory
: pass GLMemory
as immutable borrow so that no miniobject writable checks run, only the underlying data is likely to be modified.