Port of commit 548898:

Move the image transformations needed by the accessibility and
rotated viewmodes into the rendering thread. So now the GUI is
in this viewmodes as responsive as in the normal mode.

It also simplifies the code quite a bit, and and also uses only half
the memory because only one version of the page image is held
in memory.

And as an additional sideeffect, the rotated viewmodes now workagain.