[Wpf] Fix DrawingContext.CurrentTransform

The Value returned by a TransformGroup Appends each successive Transform,
even though they are Prepended when rendering.  This change evaluates the
CTM by Prepending each transform in the collection, as is required. This
restores the correct mirror-image figure in Samples.Drawing.Transformation
Merge pull request #223 from hwthomas/master

[Wpf] Fix GetCTM by correctly evaluating DrawingContext CurrentTransform