Make Futatabi fades apply white balance.

Make Futatabi fades apply white balance.

When there are two different video streams involved, they will often
have different white points, so we can't just echo the Exif data back;
we need to apply it when converting, and then get back a result in
standard sRGB. It's not entirely correct since we still run with
crushed whites/black, but it's good enough.

This completes Nageru/Futatabi white balance round trip support.