refining the avatar display name rendering with different behavior if in...

... HMD mode or not

- introduce the function "isHMDMode" in the application which tells us if we are in the HMD mode or not

- in the avatar display name rendering path, we have 2 behaviors depending on isHMDMode or not
 -- if in HMD mode, the billboard always face the mono eye of the viewer
 -- If not, always parallel to the near plane