compositor: introduce weston_surface_geometry_dirty()

Instead of directly setting the dirty flag on weston_surface geometry,
use a function for that.

This allows us to hook into geometry dirtying in a following patch.

Also add comments to weston_surface fields, whose modification causes
transform state to become outdated.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/tests/weston-test.c b/tests/weston-test.c
index b29b64f..4dc2e5c 100644
--- a/tests/weston-test.c
+++ b/tests/weston-test.c
@@ -83,11 +83,8 @@
 		wl_list_insert(&test->layer.surface_list,
 			       &surface->layer_link);
 
-	surface->geometry.x = test_surface->x;
-	surface->geometry.y = test_surface->y;
-	surface->geometry.width = width;
-	surface->geometry.height = height;
-	surface->geometry.dirty = 1;
+	weston_surface_configure(surface, test_surface->x, test_surface->y,
+				 width, height);
 
 	if (!weston_surface_is_mapped(surface))
 		weston_surface_update_transform(surface);