viewer: removed obsolete TODO.

No assert is failing anymore (or at least for the first minute of
fooling around with rotations).
cubemap: ARB_invalidate_subdata is no longer required.

If not available, the functions will fall back to no-op.
triangle: cleanup.
primitives: cleanup.
textured-triangle: cleanup, documentation improvements.
primitives: really fix the docs.

By trying to fix it I made it even worse. Gah. Development by accident.
viewer: fix docs also here.

So goddamn fragile.
Merge branch 'master' into compatibility
GCC 4.6 compatibility: can't use both const and constexpr.
GCC 4.6 compatibility: no user-defined literals.