CoverageQuadOps
Writes the unified paint vertex contract — colour, coverage (TEXCOORD2) and paint (TEXCOORD3) — into a reserved quad whose positions/UV0/UV1 were already copied from the glyph template by ReserveQuad. One writer shared by every layer kind so the contract lives in a single place.
Remarks
Fills the four destination vertices. frame maps each vertex to its paint coordinate (gradient/texture); rampRow is the gradient's ramp row (ignored for solid/texture); fade (the component colour alpha) scales every layer's alpha so fading the text fades all layers together.
Multiplies each destination vertex alpha by the GLYPH's own alpha at the source face vertex (FaceAlpha — the pre-claim stash when a fill claimed the quad, so a layer never inherits another layer's paint alpha) — the canonical formula keeping outline / shadow / extrude / textured-fill fades synchronised with face fades (per-character <alpha> ramps, ColorModifier alpha). Run after Write, whose uniform alpha carries only the component-level fade.
