Basically I want some bitmaps to scroll up and down and for any parts outside a given rectangle not to be displayed. For reason I don't want to get into, I can't simply set everything outside the rectangle to a higher z-coordinate. Also note that I'm NOT using Window_Base. I've though about this for a long time and the only solution I could think of (merging all the bitmaps and then cutting of the parts I don't want with the fill_rect command) is way too inefficient.
