Overlapped regions have transparent backgrounds, so when placing
AZONE_REGION we need to move them in to the content edge.

This patch just moves AZONE_REGION in a bit when the region is overlapped (user has enabled USER_REGION_OVERLAP so the region is draw with a transparent background) so that the hit area for dragging matches the visible edge of content.

The amount moved in is an arbitrary amount (0.4f * U.widget_unit) but does seem to work quite well:

If we need to change these in a more granular fashion we could differ by region->regiontype, but this seems good enough in my testing.