Write tile sizes into the frame header by default

[kpchoi]

This enables the tile_size_present_in_fh_flag path discussed in #228, so a
decoder can index individual tiles from the frame header without scanning the
tile data.

• The encoder now writes tile_size_in_fh[] (u(32) per tile) by default,
  reusing the existing frame header rewrite after tile encoding, in the same
  way pbu_size is backfilled
• oapve_config() gets OAPV_CFG_SET/GET_TILE_SIZE_IN_FH, settable per
  frame through OAPV_CFG_FRM(); the encoder app disables it with
  --disable-tile-size-in-fh
• The per-frame RC slot indexing in oapve_encode() now uses the loop index
  directly instead of a clamped copy, since the frame count is already
  validated at the function entry

[cpncf]

LGTM