Latest raylib 1.6

The builds refer to de865a9 commit.

...at least, directly, available if using directly audio module...

...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!

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

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

Required by gestures example....

Use instead: IsGestureDetected()

This reverts commit 7999bbafa8.

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

Internal modelview and projection matrices can be replaced before
drawing.

To avoid additional dependencies

Moved AudioError enum inside audio.c

The only thing I did not change was the _g for globals. Is there any
other way we can mark globals?

Tint color could be applied to colDiffuse... but what's the best way?
Replace it? Multiply by? A point to think about...

Functions renamed to improve library consistency

It tints all fragments, ignores lighting. Useful for some features like
feedback (damage color, ...).

Scaling normal depth (y axis) makes disappear the specular of
fragments... So I think it can be removed, it is not a very
useful/important attribute.

Draw3DCircle is useful to draw point lights radius.

- 3 light types added (point, directional, spot).
- DrawLights() function added using line shapes.
- Standard lighting example added.
- Removed useless struct variables from material and light.
- Fixed light attributes dynamic locations errors.
- Standard vertex and fragment shaders temporally added until rewrite it
as char pointers in rlgl.
TODO:
- Add normal and specular maps calculations in standard shader.
- Add control structs to handle which attributes needs to be calculated
(textures, specular...).
- Adapt standard shader to version 110.
- Rewrite standard shader as char pointers in rlgl.

- Ambient and lambert lighting added.
- Ambient and diffuse colors linked to standard shader.
- Single light linked to standard shader.
- LoadStandardMaterial() and depending functions added.

Improved materials