Use multiple threads if available for softgpu.

VIVA OVERHEAD.  But it's actually generally faster this way, even with
only 2 cores running (but not a huge improvement then.)

On an i7, this improves things ~2.5x with 8 cores.
Optimize out some clear mode checks.

Since it's really common and the rasterizer is slow for large draws.
Merge pull request #4758 from unknownbrackets/softgpu2

Use multiple threads if available for softgpu.