enable decoding of multi segment baseline images from C++ lepton

[mcroomp]

It is legal in a baseline image to encode each component separately, which C++ lepton treats the same way as a progressive image.

[github-actions]

Test Results

183 tests  +2   183 ✅ +2   25m 33s ⏱️ + 1m 6s
  4 suites ±0     0 💤 ±0 
  2 files   ±0     0 ❌ ±0 

Results for commit 4fdb84a. ± Comparison against base commit 682bc97.

This pull request removes 25 and adds 27 tests. Note that renamed tests count towards both.

lepton_jpeg_root::end_to_end ‑ verify_decode::file_08___gray2sf__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_09___grayscale__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_10___hq__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_11___iphone__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_12___iphonecity__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_13___iphonecity_with_16KGarbage__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_14___iphonecity_with_1MGarbage__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_15___iphonecrop__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_16___iphonecrop2__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_17___iphoneprogressive__
…

lepton_jpeg_root::end_to_end ‑ verify_decode::file_08___cathedral_db_non_int__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_09___cathedral_db_non_int_rustold__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_10___gray2sf__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_11___grayscale__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_12___hq__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_13___iphone__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_14___iphonecity__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_15___iphonecity_with_16KGarbage__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_16___iphonecity_with_1MGarbage__
lepton_jpeg_root::end_to_end ‑ verify_decode::file_17___iphonecrop__
…