Redesigned rlgl usage:
- rlViewport() must be called by user
- Internal projection/modelview matrices must be setup by user

Following XNA style, now we have InitGraphicsDevice(), replacing
InitDisplay() + InitGraphics()

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

- Added support for RLGL_NO_STANDARD_SHADER
- Store framebuffer width and height as globals
- Reorganize rlglInit() function

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

If Oculus device is not available or not initialized correctly,
simulated VR view is generated using stereo-rendering and distortion

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

Physac 1.0 module completed

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.

Required by gestures example....

Use instead: IsGestureDetected()

... and now physac thread creation is done in InitPhysics() and it is
destroyed in ClosePhysics(). User just need to call these functions to
use physac module.

This reverts commit 7999bbafa8.

Just started working on this, not sure if it would be available for
raylib 1.5

Old physics update system used a static number of steps to calculate
physics (450 for desktop and 64 for android). It was too much and it was
limited by target frame time...

Now physics update runs in a secondary thread using a fixed delta time
value to update steps. Collisions are perfectly detected and resolved
and performance has been improved so much.

To get it, use GetDefaultTexture()