Improve QTRLE encoding performance, no change to output file size/content.

Avoid searching for the lowest bulk cost for each pixel that isn't a repeat/skip. Instead store the lowest cost as we go along each pixel, and use it as needed.

Signed-off-by: Malcolm Bechard <malcolm.bechard@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>