Support resolution scaling and configuring main texture size of Camera

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

The main texture size of Camera is always the viewport physical size and cannot be configured, which prevents the implementation of resolution scaling/super-resolution algorithms (such as Bilinear, and AMD FSR 2.0 #5232, #12768). Resolution scaling could allow values < 1.0 for performance, or > 1.0 for supersample antialiasing.

`MainPassResolutionOverride` doesn't actually affect the main texture size. I'm not sure how it works, but it doesn't work for simple 3D scenes if I just add this component to Camera.

## What solution would you like?

Make Camera main texture size configurable. Ideally, it should be independent of the viewport and work with it, and make the final upscaling pass customizable to implement resolution scaling.

## What alternative(s) have you considered?

No

## Additional context

Godot has settings for 3D resolution scaling https://docs.godotengine.org/en/latest/tutorials/3d/resolution_scaling.html#resolution-scaling

Start by tracing how Camera determines its main texture size and how MainPassResolutionOverride currently behaves. Review the linked resolution-scaling issues and Godot's resolution-scaling documentation for context. Done means the main texture size can be configured independently of the viewport and the final upscaling pass can be customized.

Written by the indexing model from the issue text.