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!

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()

...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...

This reverts commit 7999bbafa8.

Moved all external libraries used by raylib to external folder inside
raylib src. Makefile has already been update and also the different
includes in raylib modules.

Trying to find the best way to integrate Oculus support into raylib,
making it easy for the user...

Useful to save small data files (configuration and so)
For bigger files, external data storage should be used (SDCard)

Support render texture size different than screen size

Removed BeginDrawingEx()
Added Begin2dMode() and End2dMode()

... easy code porting, transalating them to touches and gestures
internally.

Removed function SetCustomCursor(), it can be managed by the user.

I decided it is redundant and could be confusing (when mixed with 3D
drawing). It's not KISS.

Now it's possible to render to texture, old postprocessing system will
be removed on next raylib version.

...despite not being enabled on GLFW3

Depth test disabled for 2D and only used on 3D; consequently LINES vs
TRIANGLES vs QUADS buffers drawing order maters... but blending also
works ok.

Now it works ok also in RaspberryPi

To support multiple keys (including function keys)