Fix shader hang in coarse path

[raphlinus]

I'm observing a hang in the coarse path rasterizer on Nvidia 1060 (not
on other GPUs, and I think only after a more recent driver update).
Investigation shows that the hang happens when the loop to output line
segments from flattening does not terminate. I believe this is a
floating point inconsistency of some kind - the "target" should always
be at least the sum of the "val" for all quadratics, but if it
undershoots the loop may continue when it should not.

It's possible this patch is masking a deeper logic error. Ideally, I'd
instrument the shader and capture a trace of when exactly this goes
wrong. But in any case this patch shouldn't make things any worse.

[eliasnaur]

I have reports of GPU hangs (1, 2), and had hoped this change might fix them. However, I'm seeing corruption that looks similar to when I debugged paths that weren't airtightly closed.

[raphlinus]

Thanks Elias, that's good feedback. It sounds like there is indeed a logic bug, and my proposed fix looks like it's truncating paths so the coverage isn't all the way from one end to the other. Fortunately, I now have a reproducible case (at least on 1060 hardware, if not my other GPUs), so I should be able to add instrumentation to track down in more detail what's really happening. I'll not merge this PR in its current state.

[ishitatsuyuki]

This is probably superseded by #108. Would you want to close this?

[raphlinus]

Yes, thanks for the reminder.