skins2(Linux and Win): fix transparency issue at window level

skins2(Linux and Win): fix transparency issue at window level

When refreshing a window area, the window mask must first be stripped of this
 area, because the new refresh may not result in the same mask being applied.

This patch corrects the following issues :
- an animated bitmap with transparency varying from one subimage to the next
  is now guaranteed the previous subimage won't leave any trace.
- a control that becomes invisible now means transparency is restored
 if no other controls exist for the same area (instead of an undefined and visually unpleasant area)