Fix extreme performance regression when panning or zooming images.

This reverts commit ac47787f (Render raster images by using a copy of a portion of the image), aka !79 (merged). I tracked that commit down as the source of extremely bad performance when panning or zooming the image. The color correction happens every frame in a single thread on the CPU and takes upwards of 2-500ms depending on the display resolution.

The commit purports to use "QImage's improved smooth scaling", but there is no such thing; it uses Qt::SmoothTransformation, which is simple bilinear filtering, which QPainter can do on its own. It also has a crude mipmapping implementation, which in my opinion is better replaced by real mipmapping with power-of-2 levels.