[4fGradient] Veto blitters for non-opaque paints

The current blitters don't support SrcOver, so we must ensure the colors
are opaque.  Checking shader.colorsAreOpaque() is not enough as it
doesn't reflect the paint alpha.

Instead we should check for context flags & kOpaqueAlpha_Flag, which is
only set when both the color stops AND the paint are opaque.

R=reed@google.com
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Review URL: https://codereview.chromium.org/1870773002
1 file changed