Example is intended for `PLATFORM_DESKTOP` only and OpenGL 3.3 Core profile.

Now rlgl uses the `Matrix` type, just make sure it has been previously defined somewhere... I don't like this approach but it's probably the easier one for the users... still looking for a better solution... maybe using something like
`#define MATRIX_TYPE`, so it can be checked in other modules.

rlgl has been redesigned to avoid any dependency to `raylib` or `raymath`, all functions using some of those libs have been reviewed.
 - REMOVED: `Texture2D`, `Shader` structs dependency
 - REMOVED: `Vector3`, `Matrix` structs dependency
 - REMOVED: raymath functions dependency, all required math is implemented in rlgl
 - ADDED: `rlMatrix` custom rlgl type
 - ADDED: `utils.c`: `rlMatrixFromMatrix()` and `rlMatrixToMatrix()` for a safe conversion between raylib<->rlgl matrix types
 - ADDED: `rl` prefix to all `rlgl` structs
 - Other small tweaks here and there

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

* Changes to fixes based on feedback

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

Without GLFW_OPENGL_FORWARD_COMPAT, running this as a standalone will yield the error:
```
NSGL: The targeted version of macOS only supports forward-compatible core profile contexts for OpenGL 3.2 and above
```

Stumbled on this example and I love this! Adding instructions that work for me on MacOS.. Note that glfw3 will need to be statically built locally and copied to this external/libs directory on mac. I can upload my static version, but it probably makes sense to do this only if there's some general naming convention for adding platform specific folders. Like "external_osx/lib" or "external/lib_osx". Then I'll drop my static libs in there.

See #1304

Still working on the sample but almost finished. Some rlgl internals
redesign required and it will be ready!