add an option to output raw depth and an unprojection example

[yushiangw]

this PR trys to add a unprojection function following gluUnProject style

[mmatl]

@yushiangw Thanks so much for this PR! So sorry on the extreme delay, I've been quite busy and haven't had enough time for open-source. I'll review and merge shortly.

Best, Matt

[LogWell]

Hi yushiangw, do you know how to unprojection depth map to point cloud under the current version pyrender, in which the camera could be any one of "PerspectiveCamera", "OrthographicCamera", "IntrinsicsCamera" in camera. I'm not familiar with OpenGL, hope to give me a favor.