Fix iced_wgpu freezing on empty layers

The `render` method would return when an empty layer is encountered without explicitly dropping the `RenderPass` (necessary because we use `ManuallyDrop`), which would then leak memory and freeze `wgpu` until the surface was recreated.

wgpu/src/backend.rs

@@ -236,7 +236,7 @@ impl Backend {
  let bounds = (layer.bounds * scale_factor).snap();
 
  if bounds.width < 1 || bounds.height < 1 {
- return;
+ continue;
  }
 
  if !layer.quads.is_empty() {