Simplify `animviz_verify_motionpaths()` as the current code structure
got in the way of me reviewing an addition to it.
Some conditions and calculations are reordered so that they don't have
to be repeated. This allowed me to reduce nesting of the code, and make
it easier to understand when exactly a motion path is reused
cache-and-all, when it's reused but gets a new cache, and when it's
freshly allocated.
I also think this removes a theoretical memory leak, as there was a code
path that would allocate a new cache without freeing the old one. This
would require a somewhat invalid data structure to begin with, but the
code path now simply doesn't exist any more. I don't think this
problematic code path was ever hit in normal use, though.
No actual functional changes.
4d3bb6a38a