Added struct Vector4 for convenience

- Renamed enum TextureFormat to PixelFormat for consistency
- Added support for pixel format UNCOMPRESSED_R32
- Using GetPixelDataSize() where required

To be more generic and configurable

Requires some work, distortion shader could be move out of raylib to
example code...

Now it should be coherent with OpenGL math standards

Exposed some raymath useful functions to raylib API

...to avoid possible conflicting symbols

Moved to custom example, now raylib only supports simulated VR
rendering.
Oculus code was too device dependant... waiting for OpenXR.

texture.mipmaps value needs to be updated, so, texture must be passed by
reference instead of by value

...to avoid dynamic conditions on for loop (lightsCount) on standard
shader, it seems GLSL 100 doesn't support that feature... on some GPUs
like RaspberryPi...

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

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

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