// Blend func can save an operation if it knows uSrc MSB is unset.
// Untextured prims can always skip (source color always comes with MSB=0).
// For textured prims, the generic lighting funcs always return it unset. (bonus!)
// Blend func can save an operation if it knows uSrc MSB is unset.
// Untextured prims can always skip (source color always comes with MSB=0).
// For textured prims, the generic lighting funcs always return it unset. (bonus!)