Allow setting different targets for each layer of Camera

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

When rendering 2D scenes, sometimes you want to render objects placed on different layers into different targets. One example could be rendering the background and foreground into 2 separate textures and blurring background objects in post-processing. Right now this requires to setup 2 cameras and making sure that they are updated in the same way which is not very ergonomic.

## What solution would you like?

Instead of having one global target per camera, allow assigning a target to render each layer.

The issue names no files, tests, or entry points. Start by tracing how Camera layers and render targets are represented and updated, then determine how separate targets can be assigned per layer; done means background and foreground layers can render into distinct textures without requiring synchronized duplicate cameras.

Written by the indexing model from the issue text.