skins2: improve refresh of layouts

skins2: improve refresh of layouts

Up to now, notification of invalidated subareas was available for controls
but yet, the layout always ended up being totally rebuilt. For instance,
 a small animated image meant a total rebuild of everything. For simple skins,
 this could go unnoticed, but old computers had much difficulty with more
 complex skins (e.g wmp11), that were mostly busy rebuilding layouts
 over and over.

This patch ensures that only what needs to be rebuilt is rebuilt. Some skins
 like wmp11 on Linux do show dramatic improvement with this one patch.