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.
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