Reland "Add ConvertPixels versions of PremulAlphaRoundTrip"

This is a reland of b61804e94c8c1443bc64c782c4eac4f73c9c786a

Original change's description:
> Add ConvertPixels versions of PremulAlphaRoundTrip
>
> Prior to the force_highp trick, the GrConvertPixels version failed, just
> like the GPU would do if we disabled the canvas2D fast path. With the
> highp trick, all tests pass.
>
> Bug: skia:12592
> Change-Id: I63ad2fd3b67863b6a736316e7c7b3b9bd2ee8970
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467516
> Commit-Queue: Brian Osman <brianosman@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

Bug: skia:12592
Change-Id: I4c83c8d20959ab13cc493748a22fff5133706a77
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/468458
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed