Initial support for shader attribute indexing (#2546)

* Initial support for shader attribute indexing

* Support output indexing too, other improvements

* Fix order

* Address feedback

Ryujinx.Graphics.Shader/Decoders/IOpCodeAttribute.cs

@@ -1,8 +1,9 @@
 namespace Ryujinx.Graphics.Shader.Decoders
 {
-    interface IOpCodeAttribute
+    interface IOpCodeAttribute : IOpCode
     {
         int AttributeOffset { get; }
         int Count { get; }
+        bool Indexed { get; }
     }
 }