Add Begin() and End() functions to each extension for pre/post frame tasks.

Dashboard.Common/Pal/DeviceContext.cs

@@ -42,11 +42,23 @@ namespace Dashboard.Pal
             Dispose(false);
         }
 
-        public virtual void Begin() { }
+        public virtual void Begin()
+        {
+            foreach (IDeviceContextExtension extension in _extensions)
+            {
+                extension.Begin();
+            }
+        }
 
         // public abstract void Paint(object renderbuffer);
 
-        public virtual void End() { }
+        public virtual void End()
+        {
+            foreach (IDeviceContextExtension extension in _extensions)
+            {
+                extension.End();
+            }
+        }
 
         public bool IsExtensionAvailable<T>() where T : IDeviceContextExtension
         {

Dashboard.Common/Pal/IDeviceContextExtension.cs

@@ -2,5 +2,7 @@ namespace Dashboard.Pal
 {
     public interface IDeviceContextExtension : IContextExtensionBase<DeviceContext>
     {
+        void Begin() {}
+        void End() {}
     }
 }