Adding support for custom background images

This change adds support for custom background images.

Usage:
- Background images are stored in "backgrounds" folder as with jpg or png.
- Both 360 degree (equirectangular) and 6-side cubemap images are supported. This is automatically detected based on aspect ratio (with cubemap having 4:3 aspect ratio).
- Select the background from Settings.

Changes:
- The selected background image is persisted in config in this format: "background": "backgrounds/my_background.jpg",
- Image is decoded in a background thread (via Android plugin), as Texture2D.LoadImage can cause multi-second freeze on the UI thread. We then compensate for unity (re-ordering coordinate origin and also alpha channel).
- Made ground smaller & semi-transparent

Assets/Scripts/SkyboxHandler.cs.meta

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