Add render-target passthrough for picking

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

Suppose we're looking at a scene through a viewport. We want to be able to point at something in that scene with the mouse, and determine what was hit.

However, because of the indirection of the render target, picking doesn't work correctly.

## What solution would you like?

Adopt and polish [this code](https://github.com/bevyengine/bevy_editor_prototypes/pull/110/files#diff-8822a6e2ff95ec7dc42910dee18d44df639492f64b34aa91a23ae99929f24209R77). It works by duplicating the picking events onto the render target and then handling picking again.

## Additional context

This was added to the editor prototype in https://github.com/bevyengine/bevy_editor_prototypes/pull/110 by @tim-blackbird, and is a great candidate for independent upstreaming.

Start by reviewing the linked bevy_editor_prototypes pull request, especially the referenced code, then trace Bevy's picking and render-target handling. Adapt and polish the existing approach for upstream use, and verify that pointing at objects through a viewport correctly reports the picked target.

Written by the indexing model from the issue text.