[Impeller] Ignore the mask filter when saving layer

[ColdPaleLight]

According to Skia's documentation and implementation, mask filters are ignored for saved layers. So I guess Impeller needs to be consistent with it.

https://github.com/google/skia/blob/main/include/core/SkCanvas.h#L604-L605
https://github.com/google/skia/blob/main/src/core/SkCanvas.cpp#L1114

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[chinmaygarde]

IIRC @bdero added that initially. So he may have additional context. Also cc @jonahwilliams. But otherwise LGTM to match Skia behavior. Is there a reduced test case?

[bdero]

LGTM, thanks!