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.

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.

thing that is still broken is the Layer <--> Map interface.

Fixes tickets #19 and #24.

(0,0) at the bottom left.