Prevent shared RenderState mutation during face-cull flipping

[Copilot]

Mirrored geometry can require reversed face culling, but materials without additional render state currently alias shared technique or default RenderState instances. Flipping that alias permanently corrupts culling for other geometries.

• Ownership-safe flipping
  • Copy aliased render state into the material-owned mergedRenderState before mutation.
  • Preserve existing behavior when the merged state is already owned.
• Regression coverage
  • Verify mirrored geometry receives flipped culling while the shared technique state remains unchanged.

if (finalRenderState != mergedRenderState) {
    finalRenderState = mergedRenderState.copyFrom(finalRenderState);
}
finalRenderState.flipFaceCull();

• Fixes FlipFaceCull corrupts shared RenderState for backward-normal geometry #2914