Initial work to support multi-resolution images
No more font size units
Improve support for multi-resolution images
Improvements in the bitmap generation api
Bitmap generation fixes
[Mac] Fixed image rendering
[WPF] Implement hi-res icon support
[Mac] Implement ToBitmap
[Mac] Fix image rendering
Merge branch 'retina'

Conflicts:
	Xwt.WPF/Xwt.WPFBackend/FontBackendHandler.cs
	Xwt.WPF/Xwt.WPFBackend/ImageHandler.cs