Still some work left...

Replaced by example rlgl_oculus_rift

Matching the same stereo rendering result given by Oculus PC SDK for
Oculus Rift CV1 is very difficult because hardware has changed a lot and
DK2 distortion shader and parameters don't fit on CV1. Some custom
parameters have been calculated to simulate kind of CV1 stereo
rendering. Further work is required on this point.

Oculus Rift CV1 default values seem not to be available...

- Embedded VR distortion shader
- Ready to support multiple VR devices
- Fallback to VR Simulator if device not ready
- Support mono rendering over stereo rendering

Stereo rendering has been moved again to Begin3dMode() and End3dMode(),
it has some limitations but makes more sense...

Still thinking about the best way to manage this...

Now it's easier for the user! Just init Oculus device and get stereo
rendering!

It's more educational to go through new material system, so, I decide to
remove this function to avoid students confusion...

...to align it with standard raylib code. Final goal would be having the
same code work for every platform with no changes...

Under design... looking for the easiest and most comprehensive way for
the user to use VR...

and remove static from PhysicsThread().

and add instructions in physac examples to run it successful.

and added PHYSAC_NO_THREADS define. Improved physac example drawing
frames per second in screen.

texture() doesn't exist in glsl 100, it must use texture2D().