Refactor high quality texture import

* Only two texture import modes for low/high quality now:
  * S3TC/BPTC
  * ETC2/ASTC
* Makes sense given this is the general preferred and most compatible combination in most platforms.
* Removed lossy_quality from VRAM texture compression options. It was unused everywhere.
* Added a new "high_quality" option to texture import. When enabled, it uses BPTC/ASTC (BC7/ASTC4x4) instead of S3TC/ETC2 (DXT1-5/ETC2,ETCA).
* Changed MacOS export settings so required texture formats depend on the architecture selected.

This solves the following problems:

* Makes it simpler to import textures as high quality, without having to worry about the specific format used.
* As the editor can now run on platforms such as web, Mac OS with Apple Silicion and Android, it should no longer be assumed that S3TC/BPTC is available by default for it.

core/os/os.h

@@ -290,6 +290,14 @@ public:
 	virtual Vector<String> get_granted_permissions() const { return Vector<String>(); }
 
 	virtual void process_and_drop_events() {}
+
+	enum PreferredTextureFormat {
+		PREFERRED_TEXTURE_FORMAT_S3TC_BPTC,
+		PREFERRED_TEXTURE_FORMAT_ETC2_ASTC
+	};
+
+	virtual PreferredTextureFormat get_preferred_texture_format() const;
+
 	OS();
 	virtual ~OS();
 };