Shader to directly render to screen pixels

## What problem does this solve or what need does it fill?

"Pixel-perfect rendering" is a hotly requested feature for retro games, or for more unusual use cases where very-direct control is desired.

## What solution would you like?

We create a shader that does this, which is fed out of a resource that stores a screen_width x screen_height array of pixel data.

## What alternative(s) have you considered?

None, but I don't know graphics well.

## Additional context

[bevy_retro](https://github.com/katharostech/bevy_retro) by @zicklag has this functionality, but the license is very much incompatible with Bevy itself. Guidance would be lovely if you're willing to chime in <3