scripting: Cache WindowThumbnail textures

Effects like overview can create more than one thumbnail for the same
window. For example, if you have 4 virtual desktops, the overview is
going to create 9 window thumbnails for the desktop background, which in
its turn means that each thumbnail is going to have its own texture.
That's not great.

WindowThumbnailItem.sourceSize has been dropped because it's unused and
will complicate texture caching.

CCBUG: 455780