Fix VUI signaling in SPS

[kpchoi]

sps->vui_parameters_present_flag was hardcoded to 0, so none of the VUI
options (timing info, aspect ratio, color description, ...) ever reached the
bitstream. This enables it whenever any VUI field is actually requested;
default streams are unchanged.

Also fixed along the way:

• The fps-to-VUI-timing propagation was inverted (num_units_in_tick = fps.num / fps.den,
  time_scale = 1). It now propagates fps.den / fps.num, so
  -z 30000/1001 --units-in-tick 1001 signals proper 29.97 timing without
  having to spell out --time-scale.
• Several VUI command line options (--units-in-tick, --pic-struct,
  --chromaloc-tf/bf, --mv-over-pic-boundaries, --max-bits-per-cu-denom,
  --log2-max-mv-len-hor/ver, --neutral-chroma-flag, --frame-field-flag)
  were parsed into the args struct but never copied into the encoder
  parameters, i.e. silently ignored. They are wired up now.
• xeve_param_default() set videoformat to 2 (NTSC) instead of 5
  (unspecified) and left the bitstream-restriction fields at 0 instead of
  their spec defaults, which would have turned the VUI on for every stream.
• The app crashed on any early parameter-error exit (--sar 1 reproduces it
  on master) because the ERR path freed uninitialized image lists.
• --sar, --videoformat, --colorprim, etc. now also accept plain numeric
  values in addition to names.

Verified: default streams are bit-exact with master (SEI text aside); VUI is
written only when requested; timing/aspect streams decode fine with xevd;
both profiles build.

Addresses #133

[mpeg5]

LGTM