An axis-aligned rectangle in device px. Edges on whole pixels cover
exactly those pixels; fractional edges get analytic coverage, the way a
2D fillRect does. The quad only grows a one-pixel skirt for the
anti-aliased fringe when an edge is fractional, so the integer rects a
candle is made of rasterise no fragment outside themselves.
A stroked segment of half width hw between two device-px points.
round gives it the round caps a series line is stroked with (a capsule,
so two segments meeting at a point also form a round join); otherwise it
is cut square at both ends, the default butt cap. A zero-length round
segment is a dot of radius hw; a zero-length butt one is nothing.
A solid triangle: a quad whose last corner repeats, leaving one triangle degenerate.
A solid quad with a colour per corner, in strip order (the two triangles are 0-1-2 and 2-1-3), for gradient fills. No anti-aliasing: these tile edge to edge under a line that covers the seam.