If Command has z==0, then those elements won't be sorted.
Only Z !=0 will be sorted, and it will use `sort` instead of `stable_sort`
for z!=0, since it is faster
QuadCommand no longer stores a copy of the quads.
Instead it just stores a reference and the MV matrix.
Later, the Renderer when it copies the Quads to the queue, it will
convert the Quads to world coordinates
Improves SpriteTest.
It is easier to see if Sprite works as SpriteBatchNode (and vice-versa).
Needed for the new renderer.
Also adds `const` to title() and subtitle()