DebugTools: use proper sphere parameters in ShapeRenderer.
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.
Platform: added window flags to Sdl2Application.

Making resizable window the default to be consistent with GLUT and
others.
Math: Added Vector2::cross().
Mention support of both `glibc` and `newlib` NaCl toolchains.
Doc++
Primitives: added 2D/3D Line primitive.

No shit. This is usable mainly if some common code expects the
mesh wrapped in MeshData and thus it is better to just have this tiny
class instead of doing it manually.
DebugTools: 2D and 3D LineSegment support in ShapeRenderer.
Clang compilation fixes.
Use `nullptr` instead of `0`.
GCC 4.8 compilation fixes.
Minor documentation fixes.

It's not needed to have empty `()` after class name in this type
of expressions:

    (new Object())
        ->setFoo(a)
        ->setBar(b);
Use `std::` prefix everywhere.
SceneGraph: this was fixed in 59a3a7029df0524d93fe5015113c668356da93ed.
SceneGraph: fully test also Object::setClean() for list of objects.

The test fails (it doesn't clean parent objects).
SceneGraph: fixed Object::setClean().

Now also all non-clean parent objects are cleaned.