compositor: fix weston_output_transform_coordinate

weston_output_transform_coordinate() was supposed to convert from device
co-ordinates to global co-ordinates.

Commit 0f67941c broke that by converting from global to device instead,
which just magically works ok for single untransformed outputs (where the
transformation is identity)

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/compositor.c

@@ -4135,7 +4135,7 @@ weston_output_transform_coordinate(struct weston_output *output,
 		0.0,
 		1.0 } };
 
-	weston_matrix_transform(&output->matrix, &p);
+	weston_matrix_transform(&output->inverse_matrix, &p);
 
 	*x = wl_fixed_from_double(p.f[0] / p.f[3]);
 	*y = wl_fixed_from_double(p.f[1] / p.f[3]);