Fix GrUnPreMulColor.

Current version incorrectly unpacked the resulting sk unpremulled color into rgba.

BUG=skia:3208

Review URL: https://codereview.chromium.org/787713002
Revert of Make all blending up to GrOptDrawState be handled by the xp/xp factory. (patchset #7 id:140001 of https://codereview.chromium.org/759713002/)

Reason for revert:
break many gm's

Original issue's description:
> Make all blending up to GrOptDrawState be handled by the xp/xp factory.
>
> In this cl the blending information is extracted for the xp and stored in the ODS
> which is then used as it currently is. In the follow up cl, an XP backend will be added
> and at that point all blending work will take place inside XP's.
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/7c66342a399b529634bed0fabfaa562db2c0dbd4

TBR=bsalomon@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/766653008