PGMYUV seems to be always limited range. This was a format originally
invented by FFmpeg at a time when YUVJ distinguished limited from full
range YUV, and this codec never appeared to output YUVJ in any
circumstance, so hard-coding limited range preserves the status quo.

The other formats are explicitly documented to be full range RGB/gray
formats. That said, don't tag them yet, due to outstanding bugs w.r.t
grayscale formats and color range handling.

This change in behavior updates a bunch of FATE tests in trivial ways
(added tagging being the only difference).