Separate GPU engines and make state follow official docs (part 1/2) (#2422)

* Use DeviceState for compute and i2m

* Migrate 2D class, more comments

* Migrate DMA copy engine

* Remove now unused code

* Replace GpuState by GpuAccessorState on GpuAcessor, since compute no longer has a GpuState

* More comments

* Add logging (disabled)

* Add back i2m on 3D engine

Ryujinx.Graphics.Gpu/Engine/MmeShadowScratch.cs

@@ -0,0 +1,15 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.Graphics.Gpu.Engine
+{
+    [StructLayout(LayoutKind.Sequential, Size = 1024)]
+    struct MmeShadowScratch
+    {
+#pragma warning disable CS0169
+        private uint _e0;
+#pragma warning restore CS0169
+        public ref uint this[int index] => ref ToSpan()[index];
+        public Span<uint> ToSpan() => MemoryMarshal.CreateSpan(ref _e0, 256);
+    }
+}