Improve rendering speed for Gtk#'s SKWidget

This gives the `SKWidget` for Gtk# a massive performance boost by moving
the creation of the `SKSurface` (and `SKImageInfo`) to the
`CreateDrawingObjects()` method (formally `CreatePixbuf()`).  This makes
it so that these objects aren't being made every render cycle.  That is
what was causing the slowdow.

This also gets rid of the flicker reported in #534; it's flicker from
window dragging & resizing is now on par with that of Cairo's.

Another minor optimization was made, removing the call to
`base.OnExposeEvent()`.  It's not needed and the extra fuction call can
siphon off CPU cycles we'd like to keep.  Some other optimizations can
be made, but most of them probably are very minor.

closes #534

source/SkiaSharp.Views/SkiaSharp.Views.Gtk/SKWidget.cs

@@ -8,10 +8,11 @@ namespace SkiaSharp.Views.Gtk
 	public class SKWidget : global::Gtk.DrawingArea
 	{
 		private Gdk.Pixbuf pix;
+		private SKImageInfo imgInfo;
+		private SKSurface surface;
 
 		public SKWidget()
 		{
-			DoubleBuffered = false;
 		}
 
 		public SKSize CanvasSize => pix == null ? SKSize.Empty : new SKSize(pix.Width, pix.Height);
@@ -21,30 +22,23 @@ namespace SkiaSharp.Views.Gtk
 
 		protected override bool OnExposeEvent(Gdk.EventExpose evnt)
 		{
-			var result = base.OnExposeEvent(evnt);
-
 			var window = evnt.Window;
 			var area = evnt.Area;
 
 			// get the pixbuf
-			CreatePixbuf();
-			var info = new SKImageInfo(pix.Width, pix.Height, SKImageInfo.PlatformColorType, SKAlphaType.Premul);
+			CreateDrawingObjects();
 
-			// create the surface
-			using (var surface = SKSurface.Create(info, pix.Pixels, info.RowBytes))
-			{
 				// start drawing
-				OnPaintSurface(new SKPaintSurfaceEventArgs(surface, info));
+			OnPaintSurface(new SKPaintSurfaceEventArgs(surface, imgInfo));
 
-				surface.Canvas.Flush();
+			surface.Canvas.Flush();
 
-				// swap R and B
-				if (info.ColorType == SKColorType.Bgra8888)
+			// swap R and B
+			if (imgInfo.ColorType == SKColorType.Bgra8888)
+			{
+				using (var pixmap = surface.PeekPixels())
 				{
-					using (var pixmap = surface.PeekPixels())
-					{
-						SKSwizzle.SwapRedBlue(pixmap.GetPixels(), info.Width * info.Height);
-					}
+					SKSwizzle.SwapRedBlue(pixmap.GetPixels(), imgInfo.Width * imgInfo.Height);
 				}
 			}
 
@@ -52,7 +46,7 @@ namespace SkiaSharp.Views.Gtk
 			window.Clear();
 			window.DrawPixbuf(null, pix, 0, 0, 0, 0, -1, -1, Gdk.RgbDither.None, 0, 0);
 
-			return result;
+			return true;
 		}
 
 		protected virtual void OnPaintSurface(SKPaintSurfaceEventArgs e)
@@ -86,29 +80,37 @@ namespace SkiaSharp.Views.Gtk
 		{
 			if (disposing)
 			{
-				FreePixbuf();
+				FreeDrawingObjects();
 			}
 		}
 
-		private void CreatePixbuf()
+		private void CreateDrawingObjects()
 		{
 			var alloc = Allocation;
 			var w = alloc.Width;
 			var h = alloc.Height;
 			if (pix == null || pix.Width != w || pix.Height != h)
 			{
-				FreePixbuf();
+				FreeDrawingObjects();
 
 				pix = new Gdk.Pixbuf(Gdk.Colorspace.Rgb, true, 8, w, h);
+
+				// (Re)create the Skia Drawing objects
+				imgInfo = new SKImageInfo(pix.Width, pix.Height, SKImageInfo.PlatformColorType, SKAlphaType.Premul);
+				surface = SKSurface.Create(imgInfo, pix.Pixels, imgInfo.RowBytes);
 			}
 		}
 
-		private void FreePixbuf()
+		private void FreeDrawingObjects()
 		{
 			if (pix != null)
 			{
 				pix.Dispose();
 				pix = null;
+
+				// Skia objects should only exist if the Pixbuf is set as well
+				surface?.Dispose();
+				surface = null;
 			}
 		}
 	}