Shape.setTransformMv() is called for each renderer frame and for each shape,
involving 6 Matrix4f.mul() and set*() operations.

Since mutation of shape's position, rotation or scale is less frequent
than rendering one frame (for all shapes),
it is more efficient to maintain a local Matrix4f and update it 
on such single mutations.

Rendering then only needs to perform one Matrix4f.mul() operation
using this internal matrix.

Also change name from setTransformMv(PMVMatrix4f) to transformMvTo(PMVMatrix4f),
as its name might be misleading.

+++

Group/Scene's uses temp arrays for Z oder sorting,
which should be maintained locally to avoid too many temp object creations.