Mac: Add off-screen rendering support (issue #518).
- Build with the 10.7 SDK (set GYP_DEFINES='mac_sdk=10.7') to include Retina support in the cefclient OSR example.

git-svn-id: http://chromiumembedded.googlecode.com/svn/trunk@1226 5089003a-bbd8-11dd-ad1f-f1f9622dbc98