[MSAA] Artifacts in cardioid with 8xMSAA and 16xMSAA

[armansito]

The msaa8 and msaa16 AA config's introduced in #377 render with artifacts:

From @raphlinus: "I think the artifacts you're seeing are because of only 4 bits for winding number accumulation, and that test case is a good one for slamming lots of winding number increments into a small area. I'm not 100% sure though. An obvious way to validate that would be to try an 8 bit accumulator."

[raphlinus]

The swar8 branch has 8 bit per sample winding number accumulation in msaa8 mode, and resolves these artifacts, confirming that it was an overflow.

There is a performance degradation (not carefully measured, but enough to be significant). The plan is to detect the number of path segments in the slice: if <= 7, then use 4 bit accumulation because overflow can't happen, otherwise 8.