glwindow: move g_main_context_push/pop_thread_default() to run()

glwindow: move g_main_context_push/pop_thread_default() to run()

Calling g_main_context_push_thread and then g_main_context_invoke()
(used by gst_gl_window_send_message_async()) in the same thread will
cause the invoked function to run immediately instead of being delayed.

This had implications for the creation of the OpenGL context not waiting
until the main loop had completely started up and as a result would
sometimes deadlock in short create/destroy scenarios.

https://bugzilla.gnome.org/show_bug.cgi?id=775171