compositor: split weston_surface_update_transform()

Split two helper functions out of weston_surface_update_transform() to:
- make the code clearer
- update the bounding box properly even if transformation fails
- unify the return point

Also add a comment on what matrix.d[12] is.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

src/compositor.c

@@ -280,25 +280,9 @@ surface_compute_bbox(struct weston_surface *surface, int32_t sx, int32_t sy,
 				  max_x - min_x + 1, max_y - min_y + 1);
 }
 
-WL_EXPORT void
-weston_surface_update_transform(struct weston_surface *surface)
+static void
+weston_surface_update_transform_disable(struct weston_surface *surface)
 {
-	struct weston_matrix *matrix = &surface->transform.matrix;
-	struct weston_matrix *inverse = &surface->transform.inverse;
-	struct weston_transform *tform;
-
-	if (!surface->geometry.dirty)
-		return;
-
-	surface->geometry.dirty = 0;
-
-	pixman_region32_fini(&surface->transform.boundingbox);
-
-	/* transform.position is always in transformation_list */
-	if (surface->geometry.transformation_list.next ==
-	    &surface->transform.position.link &&
-	    surface->geometry.transformation_list.prev ==
-	    &surface->transform.position.link) {
 	surface->transform.enabled = 0;
 
 	pixman_region32_init_rect(&surface->transform.boundingbox,
@@ -306,11 +290,18 @@ weston_surface_update_transform(struct weston_surface *surface)
 				  surface->geometry.y,
 				  surface->geometry.width,
 				  surface->geometry.height);
-		return;
-	}
+}
+
+static int
+weston_surface_update_transform_enable(struct weston_surface *surface)
+{
+	struct weston_matrix *matrix = &surface->transform.matrix;
+	struct weston_matrix *inverse = &surface->transform.inverse;
+	struct weston_transform *tform;
 
 	surface->transform.enabled = 1;
 
+	/* Otherwise identity matrix, but with x and y translation. */
 	surface->transform.position.matrix.d[12] = surface->geometry.x;
 	surface->transform.position.matrix.d[13] = surface->geometry.y;
 
@@ -320,14 +311,37 @@ weston_surface_update_transform(struct weston_surface *surface)
 
 	if (weston_matrix_invert(inverse, matrix) < 0) {
 		/* Oops, bad total transformation, not invertible */
-		surface->transform.enabled = 0;
 		fprintf(stderr, "error: weston_surface %p"
 			" transformation not invertible.\n", surface);
+		return -1;
 	}
 
 	surface_compute_bbox(surface, 0, 0, surface->geometry.width,
 			     surface->geometry.height,
 			     &surface->transform.boundingbox);
+	return 0;
+}
+
+WL_EXPORT void
+weston_surface_update_transform(struct weston_surface *surface)
+{
+	if (!surface->geometry.dirty)
+		return;
+
+	surface->geometry.dirty = 0;
+
+	pixman_region32_fini(&surface->transform.boundingbox);
+
+	/* transform.position is always in transformation_list */
+	if (surface->geometry.transformation_list.next ==
+	    &surface->transform.position.link &&
+	    surface->geometry.transformation_list.prev ==
+	    &surface->transform.position.link) {
+		weston_surface_update_transform_disable(surface);
+	} else {
+		if (weston_surface_update_transform_enable(surface) < 0)
+			weston_surface_update_transform_disable(surface);
+	}
 }
 
 static void