More engine API work

UIXrender/Engine/BufferReceivedHandler.cs

@@ -0,0 +1,9 @@
+using System;
+using Microsoft.Iris.Render.Interop;
+
+namespace Microsoft.Iris.Render.Engine;
+
+// A custom (non-generic) delegate so ReadOnlySpan<byte> is legal here -- Span<T> can't
+// be an Action<T>/Func<T> type argument (ref structs aren't valid generic arguments),
+// but a hand-declared delegate type can take one directly.
+public delegate void BufferReceivedHandler(ContextID sourceContext, RENDERHANDLE bufferHandle, BufferFlags flags, ReadOnlySpan<byte> data);