Improved arc drawing tests
[Mac] Fixed image I/O issues
[Mac] Removed image format check that doesn't seem to work well
Merge remote-tracking branch 'origin/master' into visual-render

Conflicts:
	Testing/Tests/ReferenceImageManager.cs
More drawing tests
Implemented IDisposable for Pattern classes.
Merge pull request #164 from mkrueger/disposepatterns

Implemented IDisposable for Pattern classes.
Fixed ImagePattern in Mac and More tests
Merge remote-tracking branch 'origin/master' into visual-render

Conflicts:
	Xwt.WPF/Xwt.WPFBackend/GradientBackendHandler.cs
One more drawing test
[WPF] Fix several drawing issues with arcs
[WPF] Add missing reference test images
Improve checking of test images
Merge branch 'master' into visual-render
[WPF] Add test image