`transparent: true` works only partially (white background at start) when loading URI

Currently, transparency doesn't work before the URI has been loaded.

Cause: `PbrBundle` will contain a blended material at startup. However, the default color is RGB(A?) 1,1,1,1. Since colors are multiplied with texture, the color should probably changed at first texture update.

```rust
StandardMaterial {
  alpha_mode: AlphaMode::Blend,
  unlit: true,
  ..Default::default()
}
```