glx: make sure the context is really destroyed

GLX context is released when it's no longer current in any threads, so
we have to release the context before destroy it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/glx.c

@@ -198,6 +198,7 @@ void glx_deinit(backend_t *base) {
 
 	// Destroy GLX context
 	if (gd->ctx) {
+		glXMakeCurrent(gd->display, None, NULL);
 		glXDestroyContext(gd->display, gd->ctx);
 		gd->ctx = 0;
 	}