video-chroma: don't crash on NULL resamplers

Make dummy resamplers for all cases and only execute the horizontal
resampler instead of crashing.

See https://bugzilla.gnome.org/show_bug.cgi?id=722742
videoconvert: tweak the scoring algorithm

Make a little table of conversions and manually score them. Use this
info to define better weights for the scoring algorithm.
give separate scores for doing changes and the impact of the change,
This allows us to avoid conversion when we can but still allow fairly
lossless changes.
The old code did not penalize GRAY conversions, PAL conversions were
punished too low and depth conversions too high.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=722656