swr: [rasterizer core] refactor thread creation

swr: [rasterizer core] refactor thread creation

Create worker pool now computes number of worker threads based on
things like topologies, etc. and creates the pool but doesn't actually
launch the threads. Instead there is a separate start thread pool
function. This allows thread resources to be constructed first before
threads start.

Signed-off-by: Tim Rowley <timothy.o.rowley@intel.com>