Remove unused background resource
Add support for chromatic aberrations in crt-screen

- add new checkerboard background (for faster effect human checking)
- add RgbMode for choosing aberration working mode
- RgbMode.RgbShift correspond to the previous (only) working mode
- RgbMode.ChromaticAberration computes the chromatic dispersion from the
center of the screen towards the sides, emulating lens refraction with some
hard-coded refraction indices
- remove stupid "vtint" setup in rebind()
- updated demo UI, only RgbMode.ChromaticAberration's parameters can be
changed