Merge branch 'c++-port-of-imagewmark-add'

* Branch commit log:
  src/Makefile.mk: check: add checks that use C++ embedding
  cxx/embed.cc, src/embed.py: fix int cast in ZOOM calculation
  cxx/imagewmark.cc: remove image comment watermarking (test) code
  cxx/imagewmark.cc: integrate imagewmark_add()
  cxx/imagewmark.hh: add CLI options for "add" command
  cxx/embed.cc: add C++ impl of `imagewmark add` for watermark embedding
	This commit reimplements watermark embedding in C++, mimicking the exact
	way the watermarks are embedded in the Python reference implementation.
	This includes full watermark embedding with spread-spectrum encoding,
	dynamic zoom adaptation, and head-room clipping.
	cxx/embed.cc: implement BGR<->YIQ conversion via matrix multiplication
	cxx/embed.cc: port command_add() (from src/embed.py)
	cxx/embed.cc: port add_watermark() (from src/embed.py∶add_wm)
	cxx/embed.cc: port compute_psnr() (from src/common.py∶psnr)
	cxx/embed.cc: port round_pixels() (from dither.py)
	cxx/embed.cc: port wm_pre_clip() (from src/embed.py)
	cxx/embed.cc: add bits_to_hex()
	cxx/embed.cc: verified m_enc to match Python function
	cxx/embed.cc: verified make_wm_mask() to match Python function
	cxx/embed.cc: verified make_wm_pattern() to match Python function
	cxx/embed.cc: verified convcode_encode() to match Python function
	cxx/embed.cc: move AddOptions to imagewmark.hh, integrate into main()
	cxx/embed.cc: move DEFAULT_KEY to imagewmark.cc
	cxx/embed.cc: convcode_encode: port convcode.encode() (from convcode.py)
	cxx/embed.cc: port wm_range()
	cxx/embed.cc: port local_mean() local_variance() compute_F()
	cxx/embed.cc: port get_bytes() to read Random like numpy.unpackbits()
	cxx/embed.cc: port parse_payload() (from src/common.py)
	cxx/embed.cc: port Options, Config, DEFAULT_KEY from Python
	cxx/embed.cc: start with C++ skeleleton
  cxx/Makefile.mk: compile cxx/embed.cc with OpenCV4 into imagewmark-cxx
  cxx/embed.cc: start with C++ skeleton
  src/common.py: remove outdated comments

Signed-off-by: Tim Janik <timj@gnu.org>

Author: tim-janik