Review round 6: clipping each cone's layer to its exact box with an
antialiased clip made the cone's own reach edge partly transparent at
fractional positions, and partial pixels stacked. The layer now reaches 2 px
past the box with a hard clip. New test: ten identical cones off the pixel
grid, unrotated and rotated, match one cone pixel for pixel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>