Materials with slow loading textures never update the bind group after loading

## Bevy version

0.15-dev

## What you did

Use a material with a large (in bytes) texture.

## What went wrong

Mesh does not render with the texture showing. Renderdoc shows there's a dummy 1x1 texture bound in the bind group.

Issue (probably):
1. Texture is slow to load
2. When preparing the material, the texture is not yet ready
3. A dummy texture is slotted into the bind group instead
4. When the texture finishes loading, the bind group is never updated

This probably applies to all types that use AsBindGroup with asset handles.