This reverts commit df56628979abc17474b001c58c28c6f3637ada9d. It causes an issue where windows remain in the background even when being drawn in the foreground.
By scaling the pixbuf when loading, existing assets can be used.
When loading window control buttons and icon as pixbufs, we just set them as the source for the cairo context used to paint them. Instead, we now convert them to cairo surfaces and scale them to the correct display density before painting them. This allows us to load higher resolution assets (i.e. at twice the size) and by explicitly setting the intended size in the theme draw_ops, we can then scale them down to fit lower resolution displays, or render them at full density for HiDPI displays.