surface-actor: Add culling offset for geometry scale

This fixes a case that was overlooked in !1036 (merged) - when we have a geometry scale > 1 and Wayland subsurfaces that have an offset to their parent surface (which is often the case when the toplevel surface includes decoration/shadows etc.), we have to add extra offset to their opaque regions so they match their 'visible' location.

This is necessary as meta_cullable_cull_out_children moves the coordinate system during culling, but does not know about geometry scale.

Also, remove the redundant check for window_actor - we only hit this code path if a window_actor culls out its children.

Fixes #1095 (closed)

Tested this with the gstreamer wayland demo, see !148 (merged), and Firefox

Edited by Robert Mader

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