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/Program.cs

@@ -198,6 +198,7 @@ namespace Ryujinx.Ava
                 {
                     "opengl" => GraphicsBackend.OpenGl,
                     "vulkan" => GraphicsBackend.Vulkan,
+                    "metal" => GraphicsBackend.Metal,
                     _ => ConfigurationState.Instance.Graphics.GraphicsBackend
                 };