This lets us get rid of m_autorelease and m_destroy and we’ll be able
to implement init_game/init_draw later, for stuff that must happen
on the render thread.

This was not a very smart class. We replace it with a bidirectional map.

The containers no longer force the user to use the ptrdiff_t type for
size information. For convenience, size() now always returns an int,
and the size_s() method offers a way to handle arrays with more than
2 billion elements. Internally, we still use ptrdiff_t, though.

Since so much code had to be changed, I took the opportunity to get
rid of capitalised accessors for classes that are lowercase.

Text objects now support character spacing (as a fraction of the character
width) and text scaling. Also, the alignment enum is now safe.

scene so that g_scene can be deprecated later.

reuse the linear operations in quaternions. Also mark some constructors
explicit to better spot coding errors.