Camera3D render flags to help with Starling integration.

Add flags to allow disabling of calls to "Context3D.clear" and "Context3D.present" during render pass to help support Alternativa3D/Starling integration.
Merge pull request #57 from ChrisDenham/patch-3

Camera3D render flags to help with Starling integration.
Merge remote-tracking branch 'refs/remotes/upstream/master'