Don't pass bounds to saveLayer call when painting ImageFilter

[harryterkelsen]

This causes the filter to apply incorrectly with certain TileModes in Skwasm.

Fixes #178028

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Code Review

This pull request addresses an issue where ImageFilter was not being applied correctly with certain TileModes in Skwasm. The fix involves passing null for the bounds parameter to saveLayer when painting an ImageFilter, which required making the bounds parameter nullable in NWayCanvas.saveLayer. A new golden test is added to verify the fix for ImageFilter.blur with TileMode.clamp.

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Changes reported for pull request #181353 at sha 739c640

[mdebbar]

One of the framework goldens (here) is failing and it could be a legit regression:

✅ macOS	✅ CanvasKit
⚠️ Skwasm (before)	⚠️ Skwasm (after)

It looks like some parts of the clipping were fixed, and some other parts broke? (you may need to open the images at full size to see it).

[harryterkelsen]

@mdebbar Actually, I think the golden changes are improvements across the board.

It only looks like regressions compared to the goldens they are now "closest" to (the ones from the native linux engine). If you look at what the goldens used to be (by clicking "diff" next to the old commits) then this is definitely an improvement with clipping on ImageFilter. However, there is definitely a lingering bug as you can see in the top corner of the Skwasm golden for "widgets.image_filter_matrix_offset" being clipped off, but it's still definitely an improvement over the old Skwasm golden for this test, which had the entire top black bar clipped off.

CanvasKit (before)	CanvasKit (after)
Skwasm (before)	Skwasm (after)

[mdebbar]

Thanks for looking into it!