Added support for CSS page break properties + bugfixes

[Lucas-C]

This PR also fixes a bug introduced in PR #1207 regarding font_size_pt set in the current graphics state when using .local_context().

Before merging this PR, this bug is visible on the master branch in the following reference files:

• test/outline/html_toc.pdf: font size is incorrect
• test/outline/html_toc_2_pages.pdf: font size is incorrect
• test/outline/html_toc_with_custom_rendering.pdf: font size is incorrect
• test/local_context_init.pdf: font size is incorrect

I did not see this regression in PR #1207 because I misread the positioning of the PDFs in the HTML page produced by scripts/compare-changed-pdfs.py: in order to avoid this to happen again, I added an explicit legend at the top of the page, explaining what are the PDFs in the left & right columns.

Finally, this PR also fixes automatic page break when an image, inserted through .write_html() without height attribute, does not have enough vertical space to be rendered on a page.

Checklist:

• [x] The GitHub pipeline is OK (green), meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• [x] A unit test is covering the code added / modified by this PR

• [x] This PR is ready to be merged

• [x] In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• [x] A mention of the change is present in CHANGELOG.md

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[Lucas-C]

If you want to review this @gmischler & @andersonhc, I'll be happy to have your feedbacks 🙂

Else I'll merge this later this week.

[Lucas-C]

@gmischler: would you like to take the time to review this PR more in-depths?