docs: Conversion of ImageInput docs to tests for the scanlines examples

[ziadkhouri]

Convert scanline C++ and Python examples from the "imageinput" chapter into tests within the "docs-examples" testsuites (#3992).

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ziadkhouri / name: Ziad Khouri (9f8b8f3, 6af37ce, 594675c)

[lgritz]

LGTM! This looks correct and passes tests! (Except for some known failures unrelated to this PR.)

Congratulations!