fix(canvas): composite live media surfaces on software pixel hosts

Media-surface textures are marked presentation_only so the deterministic reference renderer can keep screenshots, goldens, and replay pixel marks producer-independent. Software pixel hosts (GTK, iOS, Android) therefore showed only the placeholder, even though they already render every live frame through that renderer.

Add an opt-in ReferenceRenderSurface.withPresentationImages flag and enable it on the live pixel-presentation path. Deterministic screenshot and golden paths keep the placeholder. Media-surface draws bypass the scale-panel memo because their content changes every frame.