[coregraphics] Add new 'Scale' overload with 'MatrixOrder' enum

- Fixes dotnet#4698: CGAffineTransform.Scale does not work like Swift's .scaledBy(x:y:)
  (dotnet#4698)
- 'Scale' monotouch-test now covers the new overload for the new multiplication order.
- Changed the Scale test's values so we have different values for 'x0' (it was always 0 before) and so it matches the test case from the bug report.

src/CoreGraphics/CGAffineTransform.cs

@@ -106,10 +106,30 @@ public void Multiply (CGAffineTransform b)
 			x0 = a.x0 * b.xx + a.y0 * b.xy + b.x0;
 			y0 = a.x0 * b.yx + a.y0 * b.yy + b.y0;
 		}
+
+		public enum MatrixOrder {
+			Prepend = 0,
+			Append = 1,
+		}
+
+		public void Scale (nfloat sx, nfloat sy, MatrixOrder order)
+		{
+			switch (order) {
+			case MatrixOrder.Append: // The new operation is applied after the old operation.
+				this = Multiply (this, MakeScale (sx, sy)); // t' = t * [ sx 0 0 sy 0 0 ]
+				break;
+			case MatrixOrder.Prepend: // The new operation is applied before the old operation.
+				this = Multiply (MakeScale (sx, sy), this); // t' = [ sx 0 0 sy 0 0 ] * t – Swift equivalent
+				break;
+			default:
+				throw new ArgumentOutOfRangeException (nameof (order));
+			}
+		}
 
+		[Advice ("The new operation is applied after the old operation: t' = t * [ sx 0 0 sy 0 0 ].\nTo have the same behavior as the native Swift API, pass 'MatrixOrder.Prepend' to 'Scale (nfloat, nfloat, MatrixOrder)'.")]
 		public void Scale (nfloat sx, nfloat sy)
 		{
-			Multiply (MakeScale (sx, sy));
+			Scale (sx, sy, MatrixOrder.Append);
 		}
 
 		public static CGAffineTransform Scale (CGAffineTransform transform, nfloat sx, nfloat sy)

tests/monotouch-test/CoreGraphics/AffineTransformTest.cs

@@ -140,15 +140,27 @@ public void StaticMultiply ()
 		[Test]
 		public void Scale ()
 		{
-			var transform = CGAffineTransform.MakeTranslation (0, 200);
-			transform.Scale (1, -1);
-
-			Assert.AreEqual ((nfloat) 1, transform.xx);
-			Assert.AreEqual ((nfloat) 0, transform.yx);
-			Assert.AreEqual ((nfloat) 0, transform.xy);
-			Assert.AreEqual ((nfloat) (-1), transform.yy);
-			Assert.AreEqual ((nfloat) 0, transform.x0);
-			Assert.AreEqual ((nfloat) (-200), transform.y0);
+			var transform1 = CGAffineTransform.MakeTranslation (1, 2);
+			// t' = t * [ sx 0 0 sy 0 0 ]
+			transform1.Scale (3, 4); // MatrixOrder.Prepend by default
+
+			Assert.AreEqual ((nfloat) 3, transform1.xx);
+			Assert.AreEqual ((nfloat) 0, transform1.yx);
+			Assert.AreEqual ((nfloat) 0, transform1.xy);
+			Assert.AreEqual ((nfloat) 4, transform1.yy);
+			Assert.AreEqual ((nfloat) 3, transform1.x0);
+			Assert.AreEqual ((nfloat) 8, transform1.y0);
+
+			var transform2 = CGAffineTransform.MakeTranslation (1, 2);
+			// t' = [ sx 0 0 sy 0 0 ] * t – Swift equivalent
+			transform2.Scale (3, 4, CGAffineTransform.MatrixOrder.Prepend);
+
+			Assert.AreEqual ((nfloat)3, transform2.xx);
+			Assert.AreEqual ((nfloat)0, transform2.yx);
+			Assert.AreEqual ((nfloat)0, transform2.xy);
+			Assert.AreEqual ((nfloat)4, transform2.yy);
+			Assert.AreEqual ((nfloat)1, transform2.x0);
+			Assert.AreEqual ((nfloat)2, transform2.y0);
 		}
 
 		[Test]