message: Metal: fix validation error when rendering to RT without depth
diff: @@ -1224,10 +1224,11 @@ void MetalDriver::draw(backend::PipelineState ps, Handle<HwRenderPrimitive> rph)
  }
  // Set the depth-stencil state, if a state change is needed.
  - DepthStencilState depthState {
  - .compareFunction = getMetalCompareFunction(rs.depthFunc),
  - .depthWriteEnabled = rs.depthWrite,
  - };
  + DepthStencilState depthState;
  + if (depthAttachment) {
  + depthState.compareFunction = getMetalCompareFunction(rs.depthFunc);
  + depthState.depthWriteEnabled = rs.depthWrite;
  + }
  mContext->depthStencilState.updateState(depthState);
  if (mContext->depthStencilState.stateChanged()) {
  id<MTLDepthStencilState> state =
  