fix(nodes): expanded masks not 100% transparent outside the fade out region

[psychedelicious]

Summary

The polynomial fit isn't perfect and we end up with alpha values of 1 instead of 0 when applying the mask. This in turn causes issues on canvas where outputs aren't 100% transparent and individual layer bbox calculations are incorrect.

Related Issues / Discussions

I actually noticed this in testing but convinced myself I was seeing things:

• Inpaint a region
• Accept the inpainted region as new raster layer
• Toggle raster layer off and on - the area around the raster layer changes brightness subtly

The impact on bbox was noted here:k
https://discord.com/channels/1020123559063990373/1149506274971631688/1355010785042104503

QA Instructions

• Inpaint a region
• Accept the inpainted region as new raster layer
• Toggle raster layer off and on - the area around the raster layer should not change at all
• The new raster layer's blue box should be the right size - not the same size as bbox

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)