docs: Convert examples within the imagebufalgo chapter.

- Update the run.py scripts to use the 'test_source_dir' variable
  instead of a relative path when copying common images, to ensure tests
  can be run using ctest;
- Convert C++ and Python examples from the "common principles" and
  "pattern generation" sections into tests within the "docs-examples"
  testsuites.

Signed-off-by: Jeremy Retailleau <jeremy.retailleau@gmail.com>