fixed recovery of metric depth for orthographic cameras

[andreanicastro]

Following the dissection of the opengl camera matrix I fixed what I think is a bug in the recovery of the metric depth for orthographic cameras. Please let me know if there are any changes or test you would like to be added to the PR.

[kampelmuehler]

This indeed fixes the issues with incorrect depth values when using orthographic cameras.

[lizelive]

this fixes orthographic depth bug @mmatl i see you have branch https://github.com/mmatl/pyrender/commits/depth_linearization