PBR meshes flicker due to delay when updating `Handle<StandardMaterial>` component

## Bevy version

24b21ea35ed58035707c1e70159a24a009102fab

## Operating system & version

MacOS 12.1

## What you did

My app can update the `Handle` component quite often, because it uses a dynamically generated texture.

## What you expected to happen

The previous render did not flicker.

## What actually happened

Meshes flicker when updated for about half of the runs, for as long as the app runs.

## Additional information

None yet.