Only defaultShader required, set default value for vertex color
attribute if not enabled and fragColor uniform

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

...despite not being enabled on GLFW3

Redesigned to work as standalone and support fordward-compatible context
(shaders review)

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.

Requires Shader access functions review

sin(), cos() functions cached and replaced by float c99 versions sinf(),
cos()

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

Reviewed vertex type variables

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

... transparent elements

- New lighting engine module which contains new data types Light and
Material. These data types and functions facilitates making a basic 3D
iluminated program with a light and a model.

- Added lighting engine module example (currently included in raylib.h;
it might be compiled by separate and include lighting.h in game source C
file).

- Corrected some opengl defines control structures and added some TODO
to fix raylib-opengl 1.1 source build (note: now source can be compiled
without errors, but rlglReadPixels() won't work properly).

Note: most of functions of phong version 330 shader are not in v100
shaders, so I couldn't write a version 100 phong shader. These functions
are included from version 150.

- New model and view transformation matrix added, useful for shaders.
Modelview matrix not deleted to keep opengl 1.1 pipeline compatibility.

- New extra function added DrawModelWiresEx() to set a rotation and
scale transformations to a wire model drawing.

- Other writing and little audio.c bug fixed.

Updated functions:
Image GetTextureData(Texture2D texture);
void *rlglReadTexturePixels(Texture2D texture);

LoadTextureEx() - Simplified parameters
UpdateTexture() - Added, allows updating GPU texture data

- Renamed function rlEnableFBO() -> rlEnablePostproFBO()
- Defined struct FBO
- Moved FBO creation to function: rlglLoadFBO()
- Reviewed rlglReadTexturePixels(), trying to support OpenGL ES -IN
PROGRESS-

...shapes based on LINES, TRIANGLES and QUADS. Now the calling order of
the drawing functions is respected!

strdup() is not a C99 function (it's POSIX), not available in emscripten

On OpenGL ES it was set to use GL_CLAMP_TO_EDGE wrap mode for textures.
On LoadOBJ() texture coordinates were wrongly Y-flipped

[rlgl] Extensions strings loading was redone to avoid a Segmentation
Fault on Raspberry Pi