The render bounds for an actor now include the area covered
by bibs, shadows, and any other widgets. In many cases this
area is much larger than we really want to consider for
tooltips and mouse selection.
An optional Margin is added to Selectable to support cases
like infantry, where we want the mouse area of the actor
to be larger than the drawn selection box.
Additionally, internally renamed VisualBounds to SelectionOverlayBounds to avoid confusion with RenderBounds.
This step was necessary to prevent actors with selectable area smaller than their graphics to be removed too early from ScreenMap even though part of the graphics should still be visible.
RA cruisers were a prime example, but to a lesser extent several other actors were affected as well.
This separation also serves as preparation to determine the final RenderBounds from multiple source bounds later, to fix the remaining ScreenMap issues (building 'bibs', aircraft shadows).
