Fix a double scaling issue in Y'CbCr conversion.

We multiplied by 224/219 once too many, causing some small accuracy issues.
Furthermore, we also did this for full-range Y'CbCr, which obviously is wrong.
The issue was so small that the unit tests kept on passing (its investigation
was prompted by a test that failed on AMD cards, which is a separate issue).

After this, the Rec. 601 matrices match Wikipedia exactly, both for limited
and full range. Added unit tests for this.