[API] TransformPoint/Distance additions to Drawing Context

[hwthomas]

Proposed in Issue #76. This is an initial implementation, with
a visual test added to the Samples/DrawingTransforms example.
Mac Backend is currently NotImplemented.

[lytico]

would a Matrix (like System.Drawing.Drawing2D.Matrix) be helpfull for you?

[hwthomas]

It would mean that Xwt would have to support that struct/class and the associated transforms, and the matrix returned by each backend is not in the exact same format (Cairo layout is the transpose of the GDI+ one - I'm not sure which way round the Mac NSAffineTransform is). My feeling was that this proposal provided the basic operations (which are required, for example, to get the bounds of text that is being displayed with a rotation and translation) without too much API bloat. But I'm very open to suggestions!

[lytico]

ok, I see, that Matrix should have a backend, but working without backend too, be standalone (without canvas, but with backend)
and, with a property, reflect the canvas-backend transforms, like Canvas.Matrix {get}, and maybe Canvas.Matrix {set} too.

[slluis]

The patch looks great. It's now merged. Thanks!