Clip/Clear updates and other fixes

The Graphics.SetClip/ClipBounds/Clear should now function on all platforms, with new tests.  Also added a SetClip(IGraphicsPath) override as it was not too difficult to do.

Other changes include:
- Re-added GraphicsPath.GetRoundRect (this was not intended to be removed)
- Implemented/fixed Bitmap.Clone(Rectangle) for all platforms
- New XamMac platform to build xamarin.mac apps
- GraphicsPath.Transform now functions correctly on all platforms (needs to multiply from previous transforms)
- Other misc stuff