Move replay mask rendering to native

### Description

Now that both iOS and Android pass screenshot in-memory to native Bitmap, we could also do the mask rendering in native (at least for replay - error-screenshots would need to work as they do now).

In order to do that, we would store the output of `WidgetFilter` in the `Screenshot` class and and let the consumer decide whether they want to use the `RawRGBAData`+masks info (for replay native integrations) or compose the final PNG screenshot (for error screenshots.

This would also enable us to unit-test masks produced by `WidgetFilter`. If we wanted to, we could test the native mask rendering with "golden file approach", i.e. comparing the generated image to an expected one.