Picture Timing SEI is intentionally not used in parsing of frame field information SEI.

In the existing code retrieving the picture timing SEI from HRD could return a nullptr, when the SEI has
not arrived yet. The pointer is dereferenced without checking and passed into frame field info SEI parsing
function and used for a consistency check.

This refactoring moves the check after parsing of all SEI messages.

Removed call to PU::spanMotionInfo in CABACReader::prediction_unit() as useless and would harm parallel processing.

Also renamed 'numReorderPics' variables to 'maxNumReorderPics'

Removed the macro JVET_R0249_SE_PREFIXES since the changes in this merge request only comprises renaming syntax elements and variables.

- rewrite xGetRefPic() and xGetLongTermRefPic() to return a nullptr, if picture is not found
- run conformance checks only on existing reference pictures
- reset picture based variables if RASL picture is skipped

JVET-R0249: Prefix syntax elements in the VPS, SPS, PPS, PH, and SH with vps_, sps_, pps_, ph_ and sh_ and rename ph_disable_xxx_flag to ph_xxx_disabled_flag.

For the syntax elements for which a prefix was added it was also made sure in this commit that the name of the syntax element aligns with the specification. For instance was rpl1_copy_from_rpl0_flag renamed to sps_rpl1_same_as_rpl0_flag.

Set default values when the bitstream does not contain any VPS

Indeed m_vps is equal to nullptr when parsing the first VPS

fix ticket 1320: Missing the process of decoding NAL_UNIT_FD when filler data NAL units are in the bitstream

- create a "dummy" VPS with ID 0 on first access in ParameterSetManager
- actually use the "dummy" VPS
- improve DecLib::m_vps handling (may still fail if multiple VPSs are in the bitstream)