fix: snap float boundary segments to integer in all_touched rasterize (fixes #3384)

[gaoflow]

Under all_touched=True, the dask tile workers recompute
world-to-pixel coordinates per tile, which can nudge on-grid boundary
segments by a few ULPs (e.g. 15.0 → 14.999999999999996). The supercover
walk then misses the correct cell row/col, diverging from the eager path.

This snaps the float pixel coordinates to the nearest integer when they
are within 8× machine epsilon (relative to coordinate magnitude), making
the dask+numpy and dask+cupy backends match the eager path pixel-for-pixel.

Fixes all merge modes (first, last, max, min, sum, count). No effect on
all_touched=False or off-grid polygons.

[github-actions]

Hi @gaoflow, thanks for the PR!

Would you mind filing a quick New contributor introduction issue when you get a chance? It helps us point you at issues that fit what you'd like to work on. Most fields are optional.

Reviewing your PR doesn't depend on it, just a friendly nudge.

[brendancol]

hey @gaoflow thanks for the PR. Unfortunately this PR is redundant with #3386

We are looking for new contributors, so if fill out a new contributor introduction issue and let's connect on more xarray-spatial stuff!

[gaoflow]

Thanks for the heads-up. I missed #3386; this is redundant, so I’ll leave this closed. I’ll open the contributor introduction before picking up more xarray-spatial work.