Route draw_rounded_frame through a new fill_round_rect_aa that fills the
straight interior solid and 4x4-supersamples only the four corner arcs,
so button/text-field/panel/list-frame corners are smooth instead of
pixelated, at negligible cost.
Consolidate the checkbox's private AA fill/blend helpers onto the shared
aa_blend_px / fill_round_rect_aa primitives.
Co-Authored-By: Claude Opus 4.8 <[email protected]>