Matthew Waters authored Feb 26, 2016 and Tim-Philipp Müller committed Dec 09, 2017

Intended for use with wrapped contexts that are created shared with gst's
gl contexts in order to manage the internal sharegroup state correctly.

e.g. with caopengllayer (which is used in glimagesink and caopengllayersink
on OS X), we create a CGL context from the gst context and the sharing state
was not being correctly set on either GL context and gst_gl_context_is_shared()
was always returning FALSE.

With 11fb4fff80b63b9d67a731d4bb238b6c0a29d774 only flushing with multiple
shared contexts, the required flush was not occuring causing screen
corruption or stuttering.

Note: this didn't affect GST_GL_API=opengl pipelines

https://bugzilla.gnome.org/show_bug.cgi?id=762620