correction to requirements install Pillow with python3
keep ref to Mouse in Display to allow thread.stop() in Display.destroy()
default lens in Camera.__init__ wrong and not the same as defaultinstance lens, corrected
oops self.lens only being set if argument lens=None
Buffer now copes with missing normals (without necessarily generating them) and missing uv coordinates. i.e. for mat_flat, mat_light shaders or for POINTS rendering