Where subgroup ops are not available, they are emulated as such:

Code with subgroup ops:

    float lH = subgroupQuadSwapHorizontal(layer);
    float lV = subgroupQuadSwapVertical(layer);
    float lD = subgroupQuadSwapDiagonal(layer);

    bool isHelperH = subgroupQuadSwapHorizontal(gl_HelperInvocation);
    bool isHelperV = subgroupQuadSwapVertical(gl_HelperInvocation);

    if (gl_HelperInvocation)
    {
        layer = !isHelperH ? lH : !isHelperV ? lV : lD;
    }

Emulated code:

    float nonHelperLayer = gl_HelperInvocation ? 0.0 : layer;
    float lH = abs(dFdxFine(nonHelperLayer));
    float lV = abs(dFdyFine(nonHelperLayer));
    float lD = abs(dFdxFine(lV));

    float isHelperDiffH = abs(dFdxFine(float(gl_HelperInvocation)));
    bool isNonHelperH = isHelperDiffH > 0.5;

    float isHelperDiffV = abs(dFdyFine(float(gl_HelperInvocation)));
    bool isNonHelperV = isHelperDiffV > 0.5;

    if (gl_HelperInvocation)
    {
        layer = isNonHelperH ? lH : isNonHelperV ? lV : lD;
    }

Both paths are supported as on nvidia devices the emulated code
misbehaves.  This change therefore effectively only enables seamful cube
map emulation on Android where subgroup operations are not supported.

Bug: angleproject:3243
Bug: angleproject:3732
Change-Id: I9664d9760756758748183eb121c626f176789f3a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1742222Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>