fixup! IN PROGRESS: Interpolate when downsampling images

Somehow I managed to flip `videotestsrc-redblue.png` without renaming
it, instead of creating `videotestsrc-redblue-flipped.png`.

`make check-integrationtests` is now passing (but some images from
internal tests of the YouView UI are still failing).
stbt.py: Fix pylint errors