Support for out-of-source builds.

Uses environment variable to tell the program where to find resource
files.
Sharder sources are still search for in the current workind directory.

includes/learnopengl/model.h

@@ -31,7 +31,7 @@ public:
 
     /*  Functions   */
     // Constructor, expects a filepath to a 3D model.
-    Model(GLchar* path, bool gamma = false) : gammaCorrection(gamma)
+    Model(string const & path, bool gamma = false) : gammaCorrection(gamma)
     {
         this->loadModel(path);
     }