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)

The p_dest object is indeed locked by the calling method (cf mediacontrol_snapshot in src/control/mediacontrol_audio_video.c).

This reverts commit ebbde26e.