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.