Implemented SkiaSharp layer rendering

src/Artemis.Core/Plugins/LayerElement/LayerElement.cs

@@ -27,21 +27,43 @@ namespace Artemis.Core.Plugins.LayerElement
         ///     Called before rendering every frame, write your update logic here
         /// </summary>
         /// <param name="deltaTime"></param>
-        public abstract void Update(double deltaTime);
+        public virtual void Update(double deltaTime)
+        {
+        }
 
         /// <summary>
-        ///     Called before rendering, in the order configured on the layer
+        ///     Allows you to perform rendering on the surface <see cref="SKCanvas" /> before any layer-clipping is applied
+        ///     <para>Called before rendering, in the order configured on the layer</para>
         /// </summary>
-        public abstract void RenderPreProcess(ArtemisSurface surface, SKCanvas canvas);
+        /// <param name="surface">The surface the layer is rendered on</param>
+        /// <param name="canvas">The entire surface canvas</param>
+        public virtual void RenderPreProcess(ArtemisSurface surface, SKCanvas canvas)
+        {
+        }
 
         /// <summary>
-        ///     Called during rendering, in the order configured on the layer
+        ///     The main method of rendering anything to the layer. The provided <see cref="SKCanvas" /> is specific to the layer
+        ///     and matches it's width and height.
+        ///     <para>Called during rendering, in the order configured on the layer</para>
         /// </summary>
-        public abstract void Render(ArtemisSurface surface, SKCanvas canvas);
+        /// <param name="surface">The surface the layer is rendered on</param>
+        /// <param name="canvas">The layer canvas</param>
+        public virtual void Render(ArtemisSurface surface, SKCanvas canvas)
+        {
+        }
 
         /// <summary>
-        ///     Called after rendering, in the order configured on the layer
+        ///     Allows you to modify the <see cref="SKShader" /> used to draw the layer's <see cref="SKBitmap" /> on the
+        ///     <see cref="SKCanvas" />.
+        ///     <para>Called after rendering, in the order configured on the layer.</para>
         /// </summary>
-        public abstract void RenderPostProcess(ArtemisSurface surface, SKCanvas canvas);
+        /// <param name="surface">The surface the layer is rendered on</param>
+        /// <param name="bitmap">The bitmap created from the layer canvas</param>
+        /// <param name="shader">The current shader used to draw the bitmap on the surface canvas</param>
+        /// <returns>The resulting shader used to draw the bitmap on the surface canvas</returns>
+        public virtual SKShader RenderPostProcess(ArtemisSurface surface, SKBitmap bitmap, SKShader shader)
+        {
+            return shader;
+        }
     }
 }