pixman-renderer: use pre-computed inverse output matrix

We've already computer the inverse of the output matrix, so we
don't need to calculate it again here.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/pixman-renderer.c

@@ -167,7 +167,7 @@ pixman_renderer_compute_transform(pixman_transform_t *transform_out,
 	/* Set up the source transformation based on the surface
 	   position, the output position/transform/scale and the client
 	   specified buffer transform/scale */
-	weston_matrix_invert(&matrix, &output->matrix);
+	matrix = output->inverse_matrix;
 
 	if (ev->transform.enabled) {
 		weston_matrix_multiply(&matrix, &ev->transform.inverse);