This patch adds format handling code for the new operations. This entails
fully decoding a format to standardized RGB, and the inverse.
Handling it this way means we can always guarantee that a conversion path
exists from A to B without having to explicitly cover logic for each path;
and choosing RGB instead of YUV as the intermediate (as was done in swscale
v1) is more flexible with regards to enabling further operations such as
primaries conversions, linear scaling, etc.
In the case of YUV->YUV transform, the redundant matrix multiplication will
be canceled out anyways.
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).
