Add const to colorspace source arguments and dependencies.
Add fz_transform_point_xy to simplify transforming a point.

Many times, the idiom p.x = x; p.y = y; fz_transform_point() is used.
This function should simplify that use case by both initializing and
transforming the point in one call.
Simplify shade vertex preparation and remove redundant memcpy calls.
Tweak fz_eval_function fakein/out buffer copying.