Viewport improvements (#3765)

Content.Client/State/IMainViewport.cs

@@ -0,0 +1,15 @@
+using Content.Client.UserInterface;
+
+namespace Content.Client.State
+{
+    /// <summary>
+    ///     Client state that has a main viewport.
+    /// </summary>
+    /// <remarks>
+    ///     Used for taking no-UI screenshots (including things like flash overlay).
+    /// </remarks>
+    public interface IMainViewportState
+    {
+        public MainViewport Viewport { get; }
+    }
+}