double blurSigma

The blurRadius in sigmas instead of logical pixels.

See the sigma argument to MaskFilter.blur.

Source 

// See SkBlurMask::ConvertRadiusToSigma().
// <https://github.com/google/skia/blob/bb5b77db51d2e149ee66db284903572a5aac09be/src/effects/SkBlurMask.cpp#L23>
double get blurSigma => blurRadius * 0.57735 + 0.5;