fixed premultiplied colors for antialiased colors

[gaaclarke]

Previously line.frag was serving up unpremultiplied colors. This resulted in weird blending and the antialiasing not showing up in most cases.

test: existing golden will change

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[flutter-dashboard]

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[gaaclarke]

Here is this PR with [width = 10, radius = 0.5, scale = 1.0]. This is looking better but it looks horrible at [width = 2, radius = 0.5, scale = 0.2]. I think we need to come up with a better way of calculating radius dynamically.

[jonahwilliams]

LGTM (but not CI: https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8719475966540248753/+/u/test:_malioc_diff/stdout)

[auto-submit]

autosubmit label was removed for flutter/flutter/165839, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[flutter-dashboard]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

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Changes reported for pull request #165839 at sha 862d59e