1) Shader manager: Allow to override #defines programatically

when loading a shader file.
2) Conditional compilation of environmental mapping in gouraud.fs
   using 1)

src/slic3r/GUI/3DScene.hpp

@@ -611,7 +611,7 @@ struct _3DScene
     static void point3_to_verts(const Vec3crd& point, double width, double height, GLVolume& volume);
 };
 
-static constexpr float BedEpsilon = EPSILON;
+static constexpr float BedEpsilon = float(EPSILON);
 
 }