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)

Using normalized [0..1] input points

...and improved mouse gestures support
Some testing still required...

Required by ProcessGestureEvent()

Mouse input is interpreted as touches to allow mouse gestures
detection... and get an unified inputs system for all platforms!

Removed GLEW external dependency, now it works with GLAD
Kept GLEW path, just in case... detected weird behaviour when testing
with gDEBugger

Added support for single header implementation and also inline functions
support

- Added function WorldToScreen(...).
- Added world to screen example.
- Review GetMouseRay() comment.
- Removed deprecated lighting functions from raylib header.

Simplified for the user (more intuitive and clear)
Removed lighting module dependency

- Fixed GetMouseRay() function.
- rlglUnproject() unused.