This only renders the window frame once until the size of the window
changes, or some other event requires re-rendering. It is rendered
to a temporary bitmap, and then the top and bottom part is stored
in one bitmap as well as the left and right part. This also adds
an opacity setting, allowing it to be rendered with a different
opacity.
This makes it easier to enhance window themes and allows using
arbitrary bitmaps with e.g. alpha channels for e.g. shadows.
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