Discard window pixmaps when the buffer size changes

Currently, we discard window pixmaps when the surface size changes, but doing so is wrong because there's no any direct relationship between the surface size and the buffer size. The surface size is a logical size and it can't be used for allocating memory for textures, etc.

I don't like how the sub-surface monitor becomes more and more bloated. In long term, I plan to introduce surface actors/surface items in the scene (as one small step towards using QtQuick's scene graph or its fork in kwin). So, the sub-surface monitor will most likely go away soon.