Redesign audio system to support multiple mix channels

Only works when raylib is installed on windows system.

texture2D() is deprecated on GLSL 330

Fix building examples on OSX

_g naming convention for globals, new error exit numbers.

Base Audio Context System

Keep asking myself why I maintain this rendering path... -___-

now it works

Not tested yet

Depending on mesh data, it can be loaded and default vertex attribute
location points are set, including colors, tangents and texcoords2

Now GLFW3 windows/context creation works ok and a sample red rectangle
has been drawn using rlgl. Next step is working in tracking
position/orientation maths and try to get a simple 3d scene...