gl: win32: fix crash when finalizing GstGLContext

gl: win32: fix crash when finalizing GstGLContext

gst_gl_context_finalize() is calling gst_gl_window_win32_quit()
which was posting a message. But then window_proc takes window's
context and get a NULL.

Now that we've got a GMainLoop we can do like other backends and
simply call g_main_loop_quit().

This also remove duplicated code to release the parent window and
potential crash there because parent_proc could be NULL if we never
created the internal window. That could happen for example if setting
state to READY then setting a window_handle, and go back to NULL state.

https://bugzilla.gnome.org/show_bug.cgi?id=749601