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.

added 21 commits

• 5e445194...5d59fadd - 16 commits from branch jvet:master
• 4a26d7d8 - (complete) syntax for scalable nesting SEI
• 7ddb4aaf - store nested buffering period SEI for parsing of nested timing SEIs
• 8317f72d - minor formatting and error message improvement
• ce0459b7 - improve error messages
• dfb21a44 - encoder code + bug fixes

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added 1 commit

• ef063c2d - fixes for MSVC build errors

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LGTM

merged

mentioned in commit 1405184a