fix(render): give degenerate segments area, pin font bit order, clarify vertex_count

include/plot/render/shaders.hpp

@@ -17,6 +17,11 @@ namespace plot::render {
 /// `plot::Rect` is top-left origin while GL is bottom-left, so the y axis is
 /// flipped exactly once, here.
 ///
+/// A degenerate segment (`a_p0 == a_p1`) keeps a finite normal and is stretched
+/// into a square mark one stroke width and fringe across, rather than
+/// collapsing to zero area and rasterising nothing — the dense path stages
+/// `min == max` buckets as exactly such segments.
+///
 /// The fragment shader discards outside the fringe and ramps alpha over the last
 /// pixel of the half width with `smoothstep`, which is what keeps a 1px trace
 /// from aliasing into dashes.