Allow fourth style of text annotation.
We now support the following four styles:
 1) in 2D screen space with size in screen space units
 2) fully in 3D space (rotates with data), sized in 3D world units
 3) billboarded in 3D (aligned to the camera), sized in 3D world units
 4) billboarded in 3D (aligned to the camera), sized in screen space units