The previous behavior is confusing if the B point is near EOF (consider
B being the duration of the file, which is strictly speaking past the
last video timestamp). The new behavior is fine as well for B being far
past EOF.
Achieve this by checking the EOF state in addition to whether playback
has reached the B point. Also, move the A-B loop code out of
command_event(). It just isn't useful anymore, and obfuscates the code
more than it makes it loop simple.
Fixes#2046.
72808be6f8