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transform state to become outdated.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/src/xwayland/window-manager.c b/src/xwayland/window-manager.c
index 9c7638f..8f0a377 100644
--- a/src/xwayland/window-manager.c
+++ b/src/xwayland/window-manager.c
@@ -823,7 +823,7 @@
 					  x - 1, y - 1,
 					  window->width + 2,
 					  window->height + 2);
-		window->surface->geometry.dirty = 1;
+		weston_surface_geometry_dirty(window->surface);
 	}
 
 	if (window->surface && !window->fullscreen) {
@@ -847,7 +847,7 @@
 			pixman_region32_fini(&window->surface->pending.opaque);
 			pixman_region32_init_rect(&window->surface->pending.opaque, 0, 0,
 						  width, height);
-			window->surface->geometry.dirty = 1;
+			weston_surface_geometry_dirty(window->surface);
 		}
 		return;
 	}