Add optional img_dpi parameter to add_media in libcairodoc.py

[Mattkmmr]

This optional parameter allows adding images with a different DPI resolutions. This is needed to fix issue #11492 that media included in PDF documents has a low quality and is blurred.
Continues #1007

[Nick-Hall]

When the add_media method is called we don't know what resolution is required for the photos. The same report could be used to generate a high quality print version or a low file size version to email someone.

The photo resolution should probably be an extra report option instead.

[lordemannd]

@Mattkmmr you marked this as a draft change, do you mean it needs more work or is it ready to be included? (BTW: I have been using this change (or previous #1007) along with addon # 285, since Jan 26, 2020 without issues)

[lordemannd]

any chance of getting this into 5.1.6 and/or 5.2?

[Mattkmmr]

@lordemannd I changed this PR to draft, because I only implemented the change for libcairodoc and textdoc. It works well for them, but the other document generation files also need an update before this PR can be added. I'll to update the rest this weekend.

[emyoulation]

@Mattkmmr Matt, is this still a 'live' issue?