Auto merge of #370 - mrobinson:edge-distance-aa-double-borders, r=glennw
Auto merge of #370 - mrobinson:edge-distance-aa-double-borders, r=glennw

Add edge-distance anti-aliasing for double borders

This includes anti-aliasing for rounded corners as well as for the
interior border edges which may fall on subpixel boundaries.
Additionally, this fixes a bug where the central gap was rendered with
white instead of transparent.

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