glcontext: track sharedness with a cookie

glcontext: track sharedness with a cookie

The previous approach of traversing the other_context weak ref tree was
1. Less performant
2. Incorrect for context destruction removing a link in the tree

Example of 2:
c1 = context_create (NULL)
c2 = context_create (c1)
c3 = context_create (c2)
context_can_share (c1, c3) == TRUE
context_destroy (c2)
unref (c2)
context_can_share (c1, c3) returns FALSE when it should be TRUE!

This does not remove the restriction that context sharedness can only
be tracked between GstGLContext's.