surface-actor: Add culling offset for geometry scale - take 2

41130b08 added a fix for culling subsurfaces with geometry scale. Unfortunately it only did so for the opaque regions, not for clip and unobscured regions, as the effect was hidden by bug that was only fixed by 3187fe8e.

Apply the same fix to clip and unobscured regions and use the chance to move most of the slightly hackish geometry scale related code into a single place.

We need to scale slightly differently in the two cases, indicated by the new ScalePerspectiveType enum, as the scale is dependent on the perspective - once from outside, once from inside of the scaled actor.

Closes #1312 (closed)