Subpixel-aware scaling

[hpjansson]

Implement the ability to scale to subpixel dimensions, i.e. 1/256th of a pixel. This should leave an appropriate amount of alpha transparency in the final row and column.

Later, we will add compositing with subpixel placement in order to have the output adjusted so the transparent edge falls on the left-hand side/on top or anywhere in between left/right and top/bottom.