Msaa none fix

addesses [issue 228](https://github.com/cinder/Cinder/issues/228), setting RendererGl( 0 ) on mac with Intel Graphics HD 3000 card.  Seems like it doesn't like that extra setting in the pixel format, although it doesn't report an error.

To be sure we're setting the pixel format correctly, I added an assert after NNSOpenGLPixelFormat is instanciated - docs say it will return nil if it could not make one with the given properties.

If you agree, I'd also like to replace the NSLog in AppImplCocoaRendererGl's init, that only checks if view is nil, with an assert.  At the time being, bad things would happen if that failed, so a debug abort / break into debugger is much preffered to a minimal log statement.  I can add it to this pull request.