NihAV: palettisation

While I’ve added palettisation support for NihAV about six years ago, it was limited to per-frame palette generation back then. Since I had only two encoders supporting paletted input and both were accepting palette changes, it worked fine. It’s only when I decided to implement MOV muxer I really got a need for palettisation using global palette, so I’ve started experimenting with that. First and foremost, design. While frame palettisation is a part of NAScale that handles video frame conversion from and to various formats, this palettisation mode is bolted to nihav-encoder. It is actually implemented in two parts: initial pass that decodes input and generates palette at the end, and actual frame palettisation (which can actually work just fine without that pass if you tell it to use some pre-defined palette). I actually started with the second part and added palette generation later (for tests using default QuickTime palette was enough). Then I went even further and extended palette…