Added ISurface.PreUpdate event (#636)

* Added ISurface.PreUpdate event

The ISurface.PreUpdate event would be used to run things like Input, which need to be updated before the user code's Update is called.

In the current state of the library, depending on how the user hooks to the events, it is possible for the input to hook to the IView.Update after the user code, which would result in the user code not seeing the latest input (until the next update).

* Changed PreUpdate event type to be just Action

Author: ThomasMiz

documentation/proposals/Proposal - Windowing 3.0.md

@@ -211,6 +211,11 @@ namespace Silk.NET.Windowing
         /// Raised when the surface is initialized for the first time.
         /// </summary>
         event Action? Created;
+        
+        /// <summary>
+        /// Raised just before the Update event is raised.
+        /// </summary>
+        event Action? PreUpdate;
 
         /// <summary>
         /// Raised when an update should be run.