EXPERIMENTAL: Metal backend (#441)

This is not a continuation of the Metal backend; this is simply bringing
the branch up to date and merging it as-is behind an experiment.

---------

Co-authored-by: Isaac Marovitz <isaacryu@icloud.com>
Co-authored-by: Samuliak <samuliak77@gmail.com>
Co-authored-by: SamoZ256 <96914946+SamoZ256@users.noreply.github.com>
Co-authored-by: Isaac Marovitz <42140194+IsaacMarovitz@users.noreply.github.com>
Co-authored-by: riperiperi <rhy3756547@hotmail.com>
Co-authored-by: Gabriel A <gab.dark.100@gmail.com>

src/Ryujinx.Common/Configuration/GraphicsBackend.cs

@@ -6,7 +6,9 @@ namespace Ryujinx.Common.Configuration
     [JsonConverter(typeof(TypedStringEnumConverter<GraphicsBackend>))]
     public enum GraphicsBackend
     {
+        Auto,
         Vulkan,
         OpenGl,
+        Metal
     }
 }