…2069)

Resolves flutter/flutter#126487.

Increases the 2-pass blur quality and distribution limit.

It turns out sigma was breaking down beyond ~255 (moderately high, but
not unreasonable for users to rely on). The Gaussian function computes
sigma^2, and half precision floats only have 5 bit exponents and
overflow for numbers above 65k.

Coincidentally, this also returns us to a state where we look a lot more
like Skia's blurs for larger blur sigmas. Medium blurs have much less
visual banding (although it's still there if you look closely). I
suspect half precision isn't really enough for tracking the integral.

Unfortunately, this means our SIMD pipelining isn't going to be as good.
I'll be interested in watching the blur-driven benchmarks for the perf
hit.

(cherry picked from commit 87a03e1)