JVET-Q0397: Scalable nesting SEI for subpictures

Note: this also contains some bug fixed for scalable nesting SEI itself. These should have gone to the refactoring MR !1627 (merged), but I didn't notice early enough and didn't want to create another MR that this one would depend upon.

• write/read number of nested SEIs (syntax bug fix)
• check SEI list after they are actually read (ordering fix)
• persistent HRD for syntax dependencies (bug fix)
• improved error messages
• subpicture related syntax (JVET-Q0397)

Bug fixes are outside the macro.