[nolight132]

Summary

• add corner_smoothing and squircle to Styled, and corner_smoothing to PaintQuad, to smooth rounded corners from circular arcs towards the continuous corners of iOS
• shape smoothed corners as superellipses that reach further along each edge and cross the diagonal where the round corner would, in the WGSL, HLSL and Metal quad shaders
• follow the smoothed outline in borders, backdrop blur and the shadow clip
• keep round corners exactly as before when smoothing is 0, and keep capsules round
• add a squircle example that steps the smoothing for fills, borders, shadows and backdrop blur
• build the example in release on macOS and Windows in CI when the shaders change, compiling Metal with xcrun and HLSL with fxc, and upload both binaries

[nolight132]

Tested on each platform.