[API] Remove ResetTransform from Drawing API and update Backends, Samples, etc

ResetTransform cannot be safely implemented in Mac CoreGraphics.
The same functionality is available using a Save/Restore pair, so it can be removed.
An unfortunate case of the Backend wagging the API!

Samples/Samples/DrawingTransforms.cs

@@ -4,6 +4,7 @@
 // Authors:
 //       Lluis Sanchez <lluis@xamarin.com>
 //       Lytico (http://limada.sourceforge.net)
+//       Hywel Thomas <hywel.w.thomas@gmail.com>
 // 
 // Copyright (c) 2011 Xamarin Inc
 // 
@@ -54,39 +55,49 @@ namespace Samples
 		
 		public virtual void Rotate (Xwt.Drawing.Context ctx, double x, double y)
 		{
-			ctx.Save ();
-			ctx.Translate (x + 30, y + 30);
-			ctx.SetLineWidth (3);
+			// draws a line along the x-axis from (0,0) to (r,0) with a constant translation and an increasing
+			// rotational component. This composite transform is then applied to a vertical line, with inverse
+			// color, and an additional x-offset, to form a mirror image figure for easy visual comparison.
+			// These transformed points must be drawn with the identity CTM, hence the Restore() each time.
 
-			// Rotation
-			
-			double end = 270;
+			ctx.Save ();	// save caller's context (assumed to be the Identity CTM)
+			ctx.SetLineWidth (3);	// should align exactly if drawn with half-pixel coordinates
+
+			// Vector length (pixels) and rotation limit (degrees)
 			double r = 30;
+			double end = 270;
 			
 			for (double n = 0; n<=end; n += 5) {
-				ctx.Save ();
+				ctx.Save ();	// save context and identity CTM for each line
+
+				// Set up translation to centre point of first figure, ensuring pixel alignment
+				ctx.Translate (x + 30.5, y + 30.5);
 				ctx.Rotate (n);
 				ctx.MoveTo (0, 0);
 				ctx.RelLineTo (r, 0);
 				double c = n / end;
 				ctx.SetColor (new Color (c, c, c));
-				ctx.Stroke ();
+				ctx.Stroke ();	// stroke first figure with composite Translation and Rotation CTM
 
-				// Visual test for TransformPoints
+				// Generate mirror image figure as a visual test of TransformPoints
 				Point p0 = new Point (0,0);
 				Point p1 = new Point (0, -r);
 				Point[] p = new Point[] {p0, p1};
-				ctx.TransformPoints (p);
-				ctx.ResetTransform ();
-				ctx.Translate (2 * r + 1, 0);
+				ctx.TransformPoints (p);	// using composite transformation
+
+				ctx.Restore ();	// restore identity CTM 
+				ctx.Save ();	// save again (to restore after additional Translation)
+
+				ctx.Translate (2 * r + 1, 0);	// extra x-offset to clear first figure
 				ctx.MoveTo (p[0]);
 				ctx.LineTo (p[1]);
 				c = 1-c;
 				ctx.SetColor (new Color (c, c, c));
-				ctx.Stroke();
-				ctx.Restore ();
+				ctx.Stroke();		// stroke transformed points with offset in CTM
+
+				ctx.Restore ();		// restore identity CTM for next line
 			}
-			ctx.Restore ();
+			ctx.Restore ();	// restore caller's context
 		}
 		
 		public virtual void Scale (Context ctx, double ax, double ay)

Samples/Samples/ScrollWindowSample.cs

@@ -90,6 +90,7 @@ namespace Samples
 		
 		protected override void OnDraw (Context ctx, Rectangle dirtyRect)
 		{
+			ctx.Save ();
 			ctx.Translate (-hscroll.Value, -vscroll.Value);
 			ctx.Rectangle (new Rectangle (0, 0, imageSize, imageSize));
 			ctx.SetColor (Xwt.Drawing.Colors.White);
@@ -97,8 +98,8 @@ namespace Samples
 			ctx.Arc (imageSize / 2, imageSize / 2, imageSize / 2 - 20, 0, 360);
 			ctx.SetColor (new Color (0,0,1));
 			ctx.Fill ();
-			ctx.ResetTransform ();
-			
+			ctx.Restore ();
+
 			ctx.Rectangle (0, 0, Bounds.Width, 30);
 			ctx.SetColor (new Color (1, 0, 0, 0.5));
 			ctx.Fill ();

Xwt.Gtk/Xwt.CairoBackend/CairoContextBackendHandler.cs

@@ -279,12 +279,6 @@ namespace Xwt.CairoBackend
 			return new Size (0,0);
 		}
 		
-		public override void ResetTransform (object backend)
-		{
-			CairoContextBackend gc = (CairoContextBackend)backend;
-			gc.Context.IdentityMatrix();
-		}
-
 		public override void Rotate (object backend, double angle)
 		{
 			CairoContextBackend gc = (CairoContextBackend)backend;

Xwt.Mac/Xwt.Mac/ContextBackendHandler.cs

@@ -271,17 +271,7 @@ namespace Xwt.Mac
 			ctx.RestoreState ();
 		}
 		
-		public override void ResetTransform (object backend)
-		{
-			// http://stackoverflow.com/questions/469505/how-to-reset-to-identity-the-current-transformation-matrix-with-some-cgcontext
-			// "Note that inverting the current CTM with CGAffineTransformInvert does not work if your current CTM is singular."
-
-			CGContext ctx = ((CGContextBackend)backend).Context;
-			CGAffineTransform matrix = ctx.GetCTM ();
-			ctx.ConcatCTM (matrix.Invert ());
-		}
-		
-		public override void Rotate (object backend, double angle)
+		public void Rotate (object backend, double angle)
 		{
 			((CGContextBackend)backend).Context.RotateCTM ((float)(angle * degrees));
 		}

Xwt.WPF/Xwt.WPFBackend/ContextBackendHandler.cs

@@ -315,13 +315,7 @@ namespace Xwt.WPFBackend
 			DrawImageCore (c.Graphics, bmp, srcRect, destRect, (float) alpha);
 		}
 
-		public override void ResetTransform (object backend)
-		{
-			var c = (DrawingContext)backend;
-			c.Graphics.ResetTransform ();
-		}
-
-		public override void Rotate (object backend, double angle)
+		public void Rotate (object backend, double angle)
 		{
 			var c = (DrawingContext)backend;
 			c.Graphics.RotateTransform ((float)angle);

Xwt/Xwt.Drawing/Context.cs

@@ -181,14 +181,6 @@ namespace Xwt.Drawing
 			handler.DrawImage (Backend, GetBackend (img), srcRect, destRect, alpha);
 		}
 
-		/// <summary>
-		/// Resets the current trasnformation matrix (CTM) to the Identity Matrix
-		/// </summary>
-		public void ResetTransform ()
-		{
-			handler.ResetTransform (Backend);
-		}
-		
 		/// <summary>
 		/// Applies a rotation transformation
 		/// </summary>