Reorganize pixel_cast

[Argoday]

pixel_cast currently requires N x N function declarations - this is a lot and needs to be better organized

Can a short-cut be designed to allow either a direct cast operator to be defined or for only a simple to / from RGB(A?)f8 and leverage a double cast for some types ??

Can Alpha <-> Non-Alpha casts be folded?

Should pixel_cast.h be split into N+1 headers?