We may cause memory leak if we only call
gdk_gl_context_finalize() doesn't call
_gdk_gl_context_destroy(). We can avoid the memory leak
by calling gdk_gl_context_destroy() instead of
g_object_unref() but this workaround has some problems.
* This isn't GLib-ish.
* We can't always call gdk_gl_context_destroy() for
doing unref gl_context. Someone may need gl_context.