OpenGL includes are ~35k lines together and it is a waste of
compilation time to include them even if they are not needed at all
(e.g. whole SceneGraph and Physics libraries). Saves ~10s of compilation
time (6:46 before, now 6:35).
It prevents unwanted implicit conversions from e.g. nullptr to Camera,
Vector2 to Physics::Point etc. By making all the constructors explicit
it is easier to routinely add the keyword to all new classes instead of
thinking about cases when to add and when not to.
Finally something that can be used for:
* Unit tests
* Resource compilation
* Benchmarks
* ...
I also desperately need something similar for Windows (and Mac OS, too).