Slight refactor of 2D curve editing

Placed common code for drawing ticks into a private method. (from Alan)
Added additional Direct2D support

1. To render to a Direct2D Windows Imaging Component (WIC) render
target.
2. D2dGeometry
3. D2dGeometrySink
(from Tom)
Added .gitignore

I copied the recommended version from GitHub for Visual Studio (
https://github.com/github/gitignore/blob/master/VisualStudio.gitignore )
and added a couple of directories specific to Authoring Tools Framework.
Fixed NullReferenceException in Clamp<T> method when value parameter is
null. (from John)
another .gitignore change

We have legitimate 'x64' and 'bin' directories, so I don't want to
ignore those.
Merge branch 'svn_trunk'