Introducing a rect_clip() function

[Lucas-C]

This is really useful to display only part of image.

Usage example:

    with pdf.rect_clip(x=..., y=..., w=..., h=...):
        pdf.image(filepath, x, y)

I can add tests if needed

[alexanderankin]

it would probably be good to add docs as well, but im not the person who can accept pull requests on this fork of the project

[Lucas-C]

Documentation added, thanks for the suggestion @alexanderankin!

[Lucas-C]

Tested added:

$ python tests/cover/test_rect_clip.py --auto --check
OK

[Lucas-C]

This was ported to fpdf2: https://github.com/alexanderankin/pyfpdf/blob/master/CHANGELOG.md#210---2020-12-07