Resample

- fix returning value from void function
- fix compiling as C
- fix some unsigned/signed comparisons;
- avoid round() since it's not in pre-C99 C;
- remove MAX_CHANNELS since I never ended up needing it;
- rename STBIR_EPSILON to STBIR_ALPHA_EPSILON;
- don't use STBIR_ALPHA_EPSILON on float input (can't remove it properly due to numeric precision, and they can do it themselves);
- optimize subtraction of STBIR_ALPHA_EPSILON;
- fix stbir__linear_to_srgb_uchar properly, plus new tests
- rewrite top-of-file documentation