glcontext/cocoa: handle sending one message during shutdown

commit ced86d61

glcontext/cocoa: handle sending one message during shutdown

gst_gl_window_quit() will attempt to send a message but will be called
from GstGLContext's finalize handler and so the weak ref that backs
gst_gl_window_get_context will return NULL as it has already been
cleared.  We need that context in send_message_async to decide whether
to run the provided callback immediately or queue in GCD

commit e5912d1f

glcontext: add context is valid precondition to get_thread()

commit 5cfefcb2

glwindow: remove unused private alive variable