workarounds a bug in KaxBlockVirtual implementation (close #1079).

- Not 100% sure about PIX_FMT_RGB4 and PIX_FMT_RGB8, because their comments in the avutil.h header are incorrect.
- We use FOURCC RGB8, but it seems we use FOURCC RGB2 for RGB8 troughout the rest of VLC. Should be checked.

ffmpeg chroma: stupid test mistake for the decoder. remove some overly "loud" errors, that are not really problems.

ffmpeg chroma: Change GetVlcChroma and GetFfmpegChroma to use a video_format_t, so that we will be able to access and set the rgb mask later on.

This is the first work required to fix #1749

Small update for png decoding library (memory leak fix mainly)

swscale: lower priority of swscale from 1000 -> 55, because it is broken for RGB formats. This way, our "native" chroma filters should take precedence. This eleviate issues with #1749 and #1762 but does NOT fix them since they are still broken for scaling.

So that our perfectionist translators get a chance to reach 100% before
we release

(It probably should have been called AquireLock then)