Revert "remove plus clamp"
Revert "remove plus clamp"

This reverts commit 539909a84a2ee9f836b2464bfc2d67dad657426a.

Reason for revert: Brian and I think ultimately clamping is saner.

Original change's description:
> remove plus clamp
> 
> Clamping for plus mode should happen only as the destination format
> requires it, or as a consequence of the limited range we're holding
> colors in the pipeline, or because non-normalized alpha is nuts.
> 
> So I've taken away the clamp on float rgb, keeping it on lowp to stay
> in its legal range, and on float alpha because it's crazy not to.
> 
> Open questions:
>   - also clamp alpha to zero?
>   - what do we do with alpha <0 or >1 in general, beyond plus?
> 
> Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
> Change-Id: I15660ce55cc393e312f18ffdc13bbfdef08ac6e0
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201040
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

TBR=14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com,brianosman@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: Iccf3fd9d2b6bf541b0e2fed4f98bf41124deeedf
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201530
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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