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.