Make decimating and non-decimating cases in SkGpuBlurUtils call into common function.
Make decimating and non-decimating cases in SkGpuBlurUtils call into
common function.

I still don't fully understand all rects and offsets and think this must
be over specified. Maybe this makes it easier to make future
simplifications? It at least strips the scale factors out of the code
that got lifted into a function.

Change-Id: I603e62b55ac9096534c36aa71959ba755087b27f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/265137
Commit-Queue: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>

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