Handle non-finite sigmas in blur image filter
Handle non-finite sigmas in blur image filter

Previously the sigma clamping code would let nans through. I considered
having SkImageFilters::Blur() return nullptr if a nan sigma was given,
but this solution protects from both initial nans and nans/infinities
produced by mapping the CTM. The result is still an un-blurred image so
behaviorally it's no different.

This also cleans up some of the sigma clamping logic redundancy and
comments in the CPU blur code.

Bug: skia:1251521
Change-Id: Ib93daac97adc3f477ce287523a0dd359a19f98af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/477296
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia