No anti-aliasing on circles filled with sharp gradients in PNG export

[MDLC01]

Description

#circle(fill: red)
#circle(fill: gradient.linear(red, red).sharp(2))

When exporting a PNG, the second circle does not have anti-aliasing:

Operating system

No response

Typst version

• I am using the latest version of Typst

[oaksprout]

Would you want this one done? — Fix Typst's PNG export so circles filled with a sharp (hard-stop) gradient keep anti-aliased edges, the same as circles with a plain fill. If yes, we'll do it and open a PR; if not, say so (or just ignore this) and we won't touch it.

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Recent examples of the same flow on other projects: stretchr/testify#1932, pypa/pip-audit#1089, sigstore/sigstore-python#1845.

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