Half-pixel upwards shift in the composite filter test and maybe elsewhere
As I specified in !70 (merged), there's what looks to be a half-pixel upwards shift, which is visible in the composite test. It got introduced somewhere in between 26e4cb46 (good) and 1366ae0f (bad). I can't bisect it because most commits in-between seem to be in a super WIP state where stuff doesn't render properly.
The example is visible on this image (for instance, the top left triangles have a purple diagonal line where they join):
The shift can also be seen by toggling between the previous image and this output from 26e4cb46: