Platform: some const fascism.
Platform: minor code cleanup in Sdl2Application.

 * Explicitly set multisample attributes in all cases (so the attributes
   are always properly set when creating another context).
 * Do both assign and test in `if` statement.
Platform: ability to negotiate context in {Glut,NaCl,Sdl2}Application.
Doc++
Explicitly mention supported platforms in README and documentation.