Fix video range in preview and DCP when source is VIDEO rather than FULL.

Fix video range in preview and DCP when source is VIDEO rather than FULL.

It looks like the handling of "video" range (16-235) sources (when
creating "full" range (0-255) outputs) has been broken for some considerable
time.  Full range outputs are used for both preview and DCP creation.

We assumed that after we called sws_setColorspaceDetails FFmpeg would take
care of the conversion from video to full, but its source suggests that
it refuses to convert to full-range RGB (only full-range YUV or
greyscale).  Previously there was a seemingly correct comment (that
FFmpeg would do nothing if source _or_ destination were RGB) but the
check on when to use video_range_to_full_range() was only activated
for RGB sources, not (also) RGB outputs.

It's hard to believe that this has been around unnoticed for so long.

https://dcpomatic.com/forum/viewtopic.php?t=1928&start=10 has a
discussion which pointed out the problem.