The Solo and Mute functionality for the NLA system should really be mutually
exclusive features. They both affect whether a given track applies or not.
The only difference is that the Solo option mutes all the others, while the Mute
only does this on a per track basis.
Before this fix, muting a strip and then making it solo meant that the solo'd
track would not play at all, which isn't really what we want.
b44201d8b9