Draft: Global shadow mapping and lighting.

This MR introduces shadow mapping for global scene shadows. The scene uses the light actors to render the shadow map. Only the first directional light actor is currently used to create the shadow map. For multiple light actors, we would need multiple shadow maps or a shadow map atlas.

This MR also changes the way Blinn-Phong lighting is calculated in the shaders. A new shader include file was created called lighting.glsl, that contains the logic to calculate Blinn-Phong based on all light actors in the scene.

This Feature requires OpenGL core profile version 4.3 minimum.

Closes #85