This loader gets the OpenGL function pointers and checks for extensions. It is an alternative to GLEW and has the advantage that it works out of the box on MacOS with a core profile and also allows only to include the defines and function declarations of a specific OpenGL version. This way the usage of 'wrong' functions can get caught during compile time (e.g. when developing for 3.2 on a machine that supports GL 4+). GLEW can still be used if desired!
The loader code is based on a modified output of glLoadGen (1.0.3), a bunch of LUA scripts that create the loader code. It's not a ready to use external library so the resulting code gets embedded directly into ACGL.

* Some GL 2.1 compatibility improvements:
  * On GL 2.1 the shaderfiles are read from a different folder by default
  * 3.0+ functions are not compiled for 2.1 any more
  * The RenderObject is not 2.1 compatible, so it's not compiled for old systems
    atm. A port could be possible.
  * ACGL compiles for GL 2.1 now
* Added compile warning in case the desired OpenGL version is not defined by
  the build system (which could screw up conditional compile)
* Minor fixes

* improved compatibility with pure core profiles
* minor typos

* Added support for direct state access based uniform setting in the ShaderProgram
* functions from the EXT_direct_state_access extension can get emulated if the extension is not present
  in this case ACGL will map the simulated versions automatically to the *EXT function pointers
  so a programmer can just call the *EXT versions and doesn't have to care if the machine the code
  will get run on does support this extension (as long a simulated version already exists)
* not all functions have emulated version right now, so check if an alternative exists before relying on
  functions from this extension in ACGL lib or implement the missing simulation function to support
  machines without this extension
* all glProgramUniform functions can be emulated (plus some others that were quick to implement)
* dummy functions for all other simulation functions exist

* OpenGL error checking level can now be set via cmake
* OpenGL header configuration works better with external defines via cmake
* OpenGL version gets checked at runtime in the ACGL::init function and complains it the version it was compiled for is not present