FCxtrans parameter fixes

- highlights: don't need to +6 row & col params, they will never be
  negative

- demosaic and hotpixels: row & col params can be -1 or -2; note this
  in comments as the rationale for +6 then modulo 6

- row/col params for FCxtrans() should be const int, not size_t:
  allows for negative numbers (to not break demosaic/hotpixels), is
  still large enough, matches type used by callers, and enforces const

- demosaic: use row & col instead of y & x to match FC() and other
  FCxtrans()