Implemented rudimental multi-display support

[Propel-antonin]

Description

• Implemented multi-display support
• Added a test scene to test multi-display support

1. I'm not using Screen.width/Screen.height anymore, instead, all GetScreenSize() methods require a displayIndex.

In build, I use something like this:

var display = Display.displays[displayIndex];
return new Vector2Int(display.renderingWidth, display.renderingHeight);

In editor, it's a little trickier, I have to cache the active editor window's resolution. If the cache for a certain displayIndex is missing, I use reflection to find the gameview windows and grab their resolution directly. This only happens once after recompiling, so it shouldn't affect performance too much.

2. This display index is obtained by first finding the canvas of a graphic, and either grabbing its displayIndex (if it's an overlay canvas) or its worldCamera's displayIndex.

Fixes #264

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Update documentations
• Others (refactoring, style changes, etc.)

Test environment

• Platform: Windows Editor, Windows Standalone
• Unity version: 6000.0.58f2 (should backport fine to 2019 versions)
• Build options: Mono

https://docs.unity3d.com/2022.2/Documentation/Manual/MultiDisplay.html
It seems like everything should still work fine on Mobile.

Checklist

• This pull request is for merging into the develop branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have checked my code and corrected any misspellings

[Propel-antonin]

This was not tested in VR yet. I do not know if Display returns the correct values using the Display API. We might need to use this in VR mode?

XRSettings.eyeTextureWidth