Add basic default multi-threading
dav1d should get a (basic) default multi-threading configuration. Implementers shouldn't have to worry about thread configurations if they don't need to. Both the
tilethreads options could use
-1 or, if it isn't used already,
0 as a parameter.
With the behavior I observed, this configuration is near-optimal, at least on x86.
--framethreads autoshould use the number of logical processors.
--tilethreads autoshould use the number of tiles the video contains, with a minimum of 2 and maximum of the number of logical processors.