Panic rendering sprite

## Bevy version

main#e018ac838d164ce00a8c8fec3748787aaddb0e05

## Operating system & version

Linux 5.10.83-1-MANJARO #1 SMP PREEMPT Wed Dec 1 14:46:38 UTC 2021 x86_64 GNU/Linux

## What you did

It happens extremely itermittently, seems to be a timing issue.

Occasionally while spawning sprites this line will panic:
https://github.com/bevyengine/bevy/blob/main/crates/bevy_sprite/src/render/mod.rs#L526

## What you expected to happen

It should probably log a warning rather than killing the app.

## What actually happened

The app panicked.

## Additional information

Seems to happen consistently on my debug builds and not on my release build. I expect the optimizations change the timing and fix the bug.

Maybe this should be another issue, but I noticed there are [~150 unwraps in the bevy codebase](https://github.com/bevyengine/bevy/search?q=unwrap).

Might be useful to address each of these scenarios and revisit whether it's worth killing the app, and if so, attach an appropriate error message indicating what happened.