Add missing call to ReleaseGraphicsResources

The Cocoa implementation of vtkOpenGLRenderWindow Finalize() method was missing a call to ReleaseGraphicsResources() and it can lead to a crash whenever an opengl context needs to be destroyed without destroying a window.

A typical example is when you reparent a QVTKWidget, see bug http://www.vtk.org/Bug/view.php?id=15840 for a full backtrace and a reproducible test case.

Simon Esneault

Cc: @ken-martin @seanm