Incorrect brush rotation logic leading to random ghost window borders while dragging

[PluMGMK]

As mentioned by Michal Necasek here, the V7VGA driver (from which this one is derived) has a bug in its non-accelerated brush-rotation code, which causes "droppings" of the selection rectangle when dragging windows. The brush-rotation code in VBESVGA.DRV was refactored and rewritten to support multi-byte pixels, but evidently I preserved the logic bug during the rewrite, because I have noticed these "droppings" too. I need to go back through the code, understand how it works, and fix it properly…