Apply linear gradient premul in 4f
Apply linear gradient premul in 4f

(spotted by reed@)

Instead of converting to SkPMColor first, apply the premul in 4f and
then just store.

A modified bench shows a significant speedup:

  17/17  MB	1	690µs	695µs	695µs	700µs	0%	█▅▁▄▆▅▁▅▆▃	8888	gradient_linear_clamp_3color
  17/17  MB	1	832µs	837µs	839µs	870µs	1%	█▁▁▂▂▂▂▂▂▂	8888	gradient_linear_clamp_hicolor
  17/17  MB	1	651µs	659µs	665µs	701µs	3%	▆█▅▁▂▂▁▁▂▂	8888	gradient_linear_clamp

vs.

  17/17  MB	1	1.03ms	1.03ms	1.04ms	1.08ms	2%	██▇▁▁▁▁▁▁▁	8888	gradient_linear_clamp_3color
  17/17  MB	1	1.17ms	1.18ms	1.18ms	1.22ms	1%	█▄▂▁▁▁▁▁▁▁	8888	gradient_linear_clamp_hicolor
  17/17  MB	1	1.1ms	1.15ms	1.14ms	1.16ms	2%	▇▇▇▇▇▇██▁▁	8888	gradient_linear_clamp

R=reed@google.com,brianosman@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2409583003

Review-Url: https://codereview.chromium.org/2409583003

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,google/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,rubenvb/skia,rubenvb/skia,google/skia