Transparent pixels in Screenshot

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

In CAD-like applications (or similar), one might want to take screenshots of the current state of the application to use in documents, in which case having a transparent background in the .png generated is good for presentations purposes.

## What solution would you like?

If the ClearColor resource is set to ClearColor::NONE (transparent background) then the screenshot would have transparent pixels in the background.

## What alternative(s) have you considered?

In my application, i've set the Tonemapping to none and the background to white to allow the screenshot to be "blended in the pdf document". While this solves the issue somehow, the meshes also get their feeling changed due to the tonemapping change.

## Additional context
Don't know if this would be too niche to be worked on, but would be a feature I surely would appreciate a lot!
If there's any guidance on how to implement that, I could certainly volunteer to implement this, but I'm kinda new to rust and bevy.

If there's no interest in such feature, feel free to close this!

Thanks!

No files or tests are named. Start by locating screenshot capture and the handling of ClearColor::NONE in Bevy's rendering path. Done means PNG screenshots preserve transparent background pixels when ClearColor is transparent.

Written by the indexing model from the issue text.