Fix panic on filter regions with coordinates outside the i32 range

[jschwe]

NonZeroRect::to_int_rect panics when the rect has coordinates outside the i32 range.
Instead, add a clamped_int_rect helper, which performs the clamping without panicking.
Adds a test, which panics before this fix.

Background: We started fuzzing servo more intensively recently, and resvg is a source of panics. This PR fixes one such panic. There are a couple more crashes pending.

[luisbg]

The panic this fixes no longer exists in main. Regression test passes.

#989 got there first, by replacing the panicking bbox.to_int_rect() with IntRect::from_xywh(...)?.

But we shouldn't close this PR because the two fixes are not equivalent:

• main (Clamp too large bboxes instead of panicking #989 ) returns None on an out-of-range rect, so the whole group is skipped and the content silently disappears.
• Fix panic on filter regions with coordinates outside the i32 range #1089 does the arithmetic in f64 and clamps to the canvas, so the visible part still renders.

Want to rewrite the PR?

[Its-Just-Nans]

linebender/tiny-skia#182 is another fix (which fix the underlying crate)

[luisbg]

The issue this PR tries to fix was resolved by #989 .

I checked by applying this PR's regression test alone to current main, with none of its source changes and it passes. The <desc> in the test SVG says "the filtered element is invisible, but the app should not panic", and that's exactly what main does now.

We should close this PR.