background-content: Anti-alias texels that intersect the circle boundary

Previously we chose to only anti-alias texels inside the boundary (clip_radius - 1.0) but zoomed in you could see it was slightly smaller than the correct curve (#2024 (closed)).

Similarly if you choose to only anti-alias texels outside that edge (clip_radius + 1.0) then you'd get an overly convex curve that doesn't match up with the straight line sections.

So now we anti-alias texels that intersect the circle boundary, regardless of which side they are mostly on. For efficiency we define "intersect" to mean any texel whose center is within 0.5 of the theoretical edge.

Fixes: #2024 (closed)

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