Backport PR #12062 on branch v3.1.x (Separate alpha and rbg interpolation then recombine to fix issue11316) - #13559

Backport PR #12062 on branch v3.1.x (Separate alpha and rbg interpolation then recombine to fix issue11316)#13559

dstansby merged 1 commit into

matplotlib:v3.1.xmatplotlib/matplotlib:v3.1.xfrom

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