This is a reland of f6e73131

Aligned BlendStateKey fields

Original change's description:
> Move sampleAlphaToCoverage out of blendState
>
> This is the second step towards exposing OES_draw_buffers_indexed (that defines independent blend state for each draw buffer). This flag is global in all graphics APIs, however D3D11 technically puts it in the blend state.
>
> D3D11: BlendStateKey was extended to keep existing D3D11 state caching semantics.
>
> D3D9: a comment was added explaining why this feature was never implemented there.
>
> Bug: angleproject:4394
> Change-Id: Ie6a294eeb6fcf4c868a1f1001c4f7efd61692ccd
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2057063
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Commit-Queue: Geoff Lang <geofflang@chromium.org>

Bug: angleproject:4394
Change-Id: Ia7aed863f0f9f6066daf1b02ecade3256f494062
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2066698
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>