Also updates the image_filter_test.dart expected color values for a blur unit test. The legacy blur algorithm would a) treat very small blur sigmas as the identity even though the effect was still perceptible and b) was very inaccurate for sigmas < 2. The unit test used a sigma of 1.0 so fell within the inaccurate range but the expected values appear to have been taken directly from the original Skia output. This updates them to match the output of the new algorithm (which is also more consistent between CPU and GPU).
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
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