AlphaDepthProgram

Alpha-tested variant of DirectionalDepthProgram for cutout-textured casters (tree billboards, fences, alpha-masked 3D-Tile submeshes): samples the caster's base-colour texture on unit 0 and discards fragments below the cutoff, so the cascade maps carry the cutout silhouette instead of the full quad. Kept separate from the opaque program - a discard-capable shader disables early depth optimisations, which opaque casters keep.