Properly write-protect segments

[DJMcNab]

Extracted from #606. Before this, we would still render even if the segments buffer was being overflowed.

I believe that this was missed in #537

[DJMcNab]

@94bryanr, does this resolve the issue you were seeing?

[94bryanr]

Unfortunately, I still see the visual artifact of nothing rendered below a certain line when the resolution is increased. This is with the new config DebugLayers set to none and MemoryHints set to Performance.

[DJMcNab]

We probably need code which reproduces this issue to be able to help

[dominikh]

Just editing the simple example to fill a shape of a large size, say 8000x8000, will demonstrate the problem when rendering to a window roughly larger than 4096x4096 (it really depends on the area). At some point, new tiles stop being rendered. Though if you're unlucky, you get a compute shader timeout instead.

If I had to guess I'd say that the limit is 256 x 256 tiles.

[DJMcNab]

Just done some experiments, and have been able to reproduce, and isolated this to us only supporting 256 bins. I've opened #680 to track this.

However, the issue @94bryanr seems to be a separate issue again, interestingly.
We need a minimal reproduction for that issue as well. I have explained why it looks like a different issue in #680.

[raphlinus]

There is a reason we don't have a check in path_tiling, though it's possible I missed something. Essentially, the number of tiles written in path_tiling should always be less than or equal to the count determined in the path_count stage. That, in turn, is checked against the buffer size in coarse, and then downstream of that, path_tiling_setup zeroes out the dispatch of path_tiling, so effectively it doesn't run.

If my analysis is correct, then we probably should close this without merging, but I am very open to an argument that something might still go wrong that this check would mitigate.

[DJMcNab]

This was discussed in #gpu > Segments and seg_counts buffer sizes. We decided that follow-up work is to make the segments and seg_counts buffer use the same capacity (even though they have different bumps).

I'll do that in a different PR.