compositor: introduce weston_surface_geometry_dirty()

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transform state to become outdated.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>