Remove "backbuffering" from PagePainter and PageView

PagePainter and PageView will sometimes do what PagePainter calls "backbuffering", which makes an extra copy of the page being drawn, resulting in a performance impact and getting in the way of any potential GPU acceleration. It appears that sometime in the past, this was required in order to do alpha blending, which is no longer the case. This merge request removes this code path, and modifies everything that used backbuffering to no longer need it.

This MR depends on (and includes) commits from !694, which was originally part of this MR, but has since been split off.