Fix OS X scrolling issue.  Closes #2078.
Merge pull request #2079 from mockingbird/fix-osx-scrolling

Fix OS X scrolling issue.  Closes #2078.
New: + CPColor checkerBoardColor for debug use.

This color has a standard 'transparency' checkerboard pattern which is especially useful for our scrolling tests where it makes scrolling easy to visualise.

Refs #2078, #2079.