Revert "app: use FAST filter when painting xfer surface"

This reverts commit 36258a67.
This commit was making the rotated canvas rendering quite horrible to
the point that I think it would make the canvas rotation feature barely
usable. See bug 759287, comments 13 to 18.
I think we will need to find other ways to accelerate rendering.
Compromise on quality is possible, but I think that in this case, this
was more than just a compromise. It was more like completely abandonning
quality. We could even see the lines "spiking" while you were rotating!
Like your drawing was alive!