message: Fix Metal error when sampler has not been bound
diff: @@ -824,6 +824,14 @@ void MetalDriver::draw(backend::PipelineState ps, Handle<HwRenderPrimitive> rph)
  samplersToBind[binding] = samplerState;
  });
  + // Assign a default sampler to empty slots, in case Filament hasn't bound all samplers.
  + // Metal requires all samplers referenced in shaders to be bound.
  + for (auto& sampler : samplersToBind) {
  + if (!sampler) {
  + sampler = mContext->samplerStateCache.getOrCreateState({});
  + }
  + }
  +
  // Similar to uniforms, we can't tell which stage will use the textures / samplers, so bind
  // to both the vertex and fragment stages.
  