Memory leak calling get_mut on texture assets resource

## Bevy version

0.5.0

## Operating system & version

Windows 10

## What you did

I'm trying to dynamically generate textures, and noticed memory usage increases whenever I was updating a texture. Turns out it happens when calling `textures.get_mut` (Thanks to TheRawMeatball on Discord for figuring that out).

## What you expected to happen

The borrowed mutable texture handle should be freed after use.

## What actually happened

Memory usage keeps increasing as the game runs.

## Additional information

I've created a repo with an example (115 LOC): https://github.com/oal/bevy-memory-leak Commenting out line 32 does nothing, but commenting out line 30-33 makes the leak go away, which means something's going wrong in the `get_mut` call.

Start by running the 115-line reproduction in https://github.com/oal/bevy-memory-leak and inspect the code around lines 30–33, especially the textures.get_mut call. Compare memory use with those lines enabled and disabled; done means repeatedly updating the texture no longer causes memory usage to keep increasing.

Written by the indexing model from the issue text.