HEVC: support of interlaced content (TFF/BFF, field height and frame rate)

[tobbee]

What

Fixes #1092

Reports interlacing for HEVC, mirroring the existing AVC behaviour. Until now HEVC
streams had no ScanType/ScanOrder, and field-sequence streams were reported with the
coded field dimensions and rate (e.g. 1920×540 @ 50) instead of the frame values.

HEVC slice headers carry no field_pic_flag/bottom_field_flag; field coding is signalled
only via metadata, all of which the parser already read and discarded, or stored:

• field_seq_flag (VUI) — was Skip_SB

• pic_struct / source_scan_type (pic_timing SEI) — were Skip_S1

• general_progressive_source_flag / general_interlaced_source_flag (profile_tier_level)

  Behaviour

• Fills ScanType, ScanOrder (TFF/BFF), ScanType_StoreMethod
  (Interleaved vs Separated) and Interlacement, from pic_struct (per Table D.2), with a
  fallback to source_scan_type and the profile_tier_level constraint flags when no
  pic_timing SEI is present.

• For separated fields (field_seq_flag=1, pic_struct ∈ {1,2,9–12}) the coded picture is a
  single field, so Height is doubled (1920×540 → 1920×1080) and FrameRate is halved
  (50 → 25) — this addresses @svart-riddare's point in Does not report interlacing on HEVC video contents #1092.

• Field order for separated fields is derived from the parity of the first coded field of the
  initial IRAP (analogous to AVC's pic_struct_FirstDetected).

  Before / after

  On a 1080i50 HEVC .ts:

  Field	Before	After
  Height	540	1080
  FrameRate	50.000	25.000
  ScanType	(absent)	Interlaced
  ScanOrder	(absent)	TFF
  ScanType_StoreMethod	(absent)	SeparatedFields

  Notes

• I can provide a 1080i50 HEVC .ts sample for the test suite.

• Out of scope: frame doubling/tripling (pic_struct 7/8) rate handling — currently reported
  as Progressive, unchanged.

  Spec

  ISO/IEC 23008-2 (H.265): Table D.2 (pic_struct), source_scan_type semantics,
  field_seq_flag semantics (incl. NOTE 11 on field dimensions/rate).