[Impeller] Make matrix image filter in saving layer work as expected

[ColdPaleLight]

fix flutter/flutter#121893
fix flutter/flutter#120879

This PR tries to distinguish whether it is a subpass scene (aka. save layer) in MatrixImageFilterContents, and do different processing for subpass scenes.

without patch

with patch

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[chinmaygarde]

Thank you so much! Minor nits but lgtm otherwise.

[bdero]

The given effect transform should already have its translation/projective coords stripped. Can we remove the Basis() calls here and in the gaussian blur?

[ColdPaleLight]

The given effect transform should already have its translation/projective coords stripped. Can we remove the Basis() calls here and in the gaussian blur?

I modified the implementation of FilterContents::SetEffectTransform, because MatrixImageFilter needs an original matrix instead of a stripped one, otherwise the result will be incorrect (such as when nesting Transformed widget). So I added Basis() to blur and morphology.

void FilterContents::SetEffectTransform(Matrix effect_transform) {
-  effect_transform_ = effect_transform.Basis();
+  effect_transform_ = effect_transform;
}