gtkglsink: re parenting the video widget into a different toplevel does not work

Submitted by Juan Pablo Ugarte

Link to original bug (#775045)

Description

Created attachment 340711
Simple test case

Reparenting the video widget from gtkglsink to a different toplevel does not work because the GdkGLContext is different and GtkGstGLWidget uses the old one.

This is needed to implement fullscreen for example.

Ideally GtkGstGLWidget should get the context in the realize event and free it on unrealize if possible.

Attached a simple test case that toggles fullscreen mode on key release

Attachment 340711, "Simple test case":
test-player.c