slicethread: Limit the automatic number of threads to 16

slicethread: Limit the automatic number of threads to 16

This matches a similar cap on the number of automatic threads
in libavcodec/pthread_slice.c.

On systems with lots of cores, this fixes a couple fate failures
in 32 bit mode on such machines (where spawning a huge number of
threads runs out of address space).

Signed-off-by: Martin Storsjö <martin@martin.st>
Gbp-Pq: Name 0002-slicethread-Limit-the-automatic-number-of-threads-to.patch