pixdesc: Improve scoring for opaque/unknown pixel formats

Hardware pixel formats do not tell you anything about their actual
contents, but should still score higher than formats with completely
unknown properties, which in turn should score higher than invalid
formats.

Do not return an AVERROR code as a score.

Fixes a hang in libavfilter where format negotiation gets stuck in a
loop because AV_PIX_FMT_NONE scores more highly than all other
possibilities.
