Clean up factory_manager a bit
Honor the color matrix tag from the subtitles file when appropriate

If the script's matrix matches the video's matrix, use that even if
Force BT.601 is on, and if it's TV.601, use that even if Force BT.601 is
off.

This should mostly eliminate the common problems resulting from  mixed
values of the Force BT.601 option.

Closes #1649.