utils.c is getting quite crowded, and I need a new place to dump a lot of
format handling code for the ongoing rewrite. Rather than bloating this file
even more, start splitting format handling helpers off into a new file.
This also renames the existing utils.h header, which didn't really contain
anything except the SwsFormat definition anyway (the prototypes for what should
have been in utils.h are all still in the legacy swscale_internal.h).
This is a lightweight wrapper around the underlying color management system,
whose job it is merely to manage the 3DLUT state and apply them to the frame
data. This is where we might add platform-specific optimizations in the future.
I also plan on adding support for more pixel formats in the future. In
particular, we could support YUV or XYZ input formats directly using only
negligible additional code in the 3DLUT setup functions. This would eliminate
the major source of slowdown, which is currently the roundtrip to RGBA64.
