qyot27: I looked at the avisynth_c.h you mentioned that has support for the new colorspaces in 2.6. What sucks is that it is neither source- nor binary-compatible with the avisynth_c.h header from upstream avisynth. So I have to the choice of 1) either keeping the original, or 2) merging features from ffms2 but breaking C-compatibility to upstream.
As a sad note, the C-interface has a bunch of baked code. What surprises me is that Avisynth now has three different APIs (2.5, 2.6, and C), and all of them suck. I honestly feel tempted to finally create an API for Avisynth that 1) doesn't rely on the representation of internal data structures, 2) doesn't rely on baked code, and 3) can be used universally from many different compilers and computer languages (right now whichever existing interface you choose, you can pick at most only one of these properties). I'm just afraid that
XKCD #927 will set in.