Operate over global variable

Removed functions from rlgl module.

Avoid RLGL data dependency to allow moving the functions somewhere else

As a bonus, stereo render is compatible with mesh instancing now!

rlgl module has been completely redesigned to move Mesh/Material structures to [models] module. Still some work to do, broken elements:
 - [models] OpenGL 1.1 mesh rendering: DrawMesh()
 - [models] Mesh Instancing: DrawMeshInstanced()
 - [models] Stereo rendering: DrawMesh()
 - [models] GL_FLOAT, GL_UNSIGNED_INT exposed
 - [models] GenMeshCustom()
 - [rlgl] GenTexture*() functions removal?

- Many functions renamed to follow rl*() convention
- Some internal functions exposed in the API
- Some functionality moved to other modules
- Reorganized all functions by categories
- Make sure it keeps working with OpenGL 1.1 and 2.1

- Some rlgl functions have been moved to core
 - Some functions have been made internal to rlgl
 - rlgl functions prefixed with rl*()

Vr simulator has been moved to core module and completely redesigned. Now user is in charge of stereo-render fbo and also in full control of distortion shader. Code is a bit more complex but better aligned with other raylib examples.

Now enum names are more consistent between them.

Just assigned a fixed memory size for custom Material parameters in case of being required, so we shouldn't worry about allocating/freeing them.

Checking for extension and enabling it if available

Some enums values have been renamed to be more consistent and also provide a more detailed description:
 - ShaderLocationIndex:    LOC_VERTEX_POSITION -> SHADER_SHADER_LOC_VERTEX_POSITION
 - ShaderUniformDataType:  UNIFORM_VEC2 -> SHADER_UNIFORM_VEC2
 - MaterialMapType: MAP_ALBEDO -> MATERIAL_MAP_ALBEDO
 - PixelFormat: UNCOMPRESSED_GRAYSCALE -> PIXELFORMAT_UNCOMPRESSED_GRAYSCALE

RENAMED: TextureFilterMode values
RENAMED: TextureWrapMode values

Updates skybox demo to show how to do both HDR and non HDR skyboxes

Co-authored-by: Jeffery Myers <JefMyers@blizzard.com>

Actually, it seems more logical that rlgl takes care of OpenGL data than the models module...

Also, models module loaded vertex data is unloaded by models module.

* Fixing typecast warnings generated by visual studio 2019 in examples.

* Changes to fixes based on feedback

Co-authored-by: Jeffery Myers <JefMyers@blizzard.com>

Sorry if this is a bit of a useless pr

Try to avoid a possible false-positive memory leak...

CMake relied on this file for configurations and also was interfering in the regular config.h by having a separate definition if building with CMake. This was not entirely correct so instead we will define compile time definitions separately through CMake (CompileDefinitions.cmake) and also will use the provided EXTERNAL_CONFIG_FLAGS that I found that will not use config.h in through the build process.

I also introduced a new compiler option (CUSTOMIZE_BUILD) that when OFF will use the default config.h and when ON will show other options for redefining your own options.

Fixed an error in rlgl.h where if you have both RLGL_STANDALONE and SUPPORT_VR_SIMULATOR you get a compile time error.

I usually try to avoid any return in the middle of functions, I try to keep them always at the end of the functions.