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/Config.cs

@@ -28,6 +28,9 @@ namespace QuestAppLauncher
         public const string DownloadRepo_Type_GitHub = "github";
         public const string DownloadRepo_Default = @"tverona1/QuestAppLauncher_Assets/releases/latest";
 
+        // Background
+        public const string Background_Default = "default";
+
         /// <summary>
         /// Grid size
         /// </summary>
@@ -66,6 +69,9 @@ namespace QuestAppLauncher
         // Whether to auto-download updates
         public bool autoUpdate = false;
 
+        // Background image path
+        public string background = Background_Default;
+
         // Github download repos
         public List<DownloadRepo> downloadRepos = new List<DownloadRepo>();