fix(mDCv): Correct the floating-point setter `png_set_mDCv`

In the floating-point API function `png_set_mDCv` we incorrectly
divided by two the chromaticity values before passing them on to
`png_set_mDCv_fixed`.

Reported-by: Mohit Bakshi <mohitbakshi2205@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>