JVET-Q0404: add a flag to subpicture info SEI to indicate constant bitrate
Includes:
- added flag
- minor updates for variable names (see discussion in !1626 (merged))
- update for trace strings according to JVET-R2001-vA
Note: VTM does not include a CBR encoder, thus the encoder sets the flag to false.
Edited by Karsten Suehring