pushOpacity method
Pushes an opacity multiplier onto the opacity stack.
Multiplicative opacity is applied to incoming Cell RGB style channels. This
enables nested translucent overlays where opacity composes as a product.
Implementation
void pushOpacity(double opacity) {
final clamped = opacity.clamp(0.0, 1.0);
_opacityStack.add(_opacityStack.last * clamped);
}