Make PageDirectory retainable and have each Region co-own the PageDirectory
they're mapped into. When unmapped, Region has no associated PageDirectory.
This allows Region to automatically unmap itself when destroyed.
Previously we'd blit every pixel in every window that intersected any dirty
rect to the back buffer. With this patch, we limit ourselves to blitting the
pixels inside the actual dirty rects.
There's still a lot of optimizations to make in this code.
There is some trouble here with the asynchronous nature of WindowServer
and the single per-window backing store we're drawing into. If we start
repainting a widget with a pending invalidation, that invalidation might
get flushed by the WindowServer mid-paint.
