[FEATURE][symbology] Add density-based point count for the random marker fill

[nirvn]

Description

This PR adds a density-based point count method to @nyalldawson 's brand new random marker fill.

GIF time:

Edit: i should probably expand a little bit more here. The benefit of this new method is that contrary to an absolute count, the fill density will remain the same on different scale / zoom levels (as demonstrated in the GIF above).

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include Fixes #11111 at the bottom of the commit message
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit
• I have evaluated whether it is appropriate for this PR to be backported, backport requests are left as label or comment

[nirvn]

@nyalldawson , I wasn't so sure of the newly-added function wording, feel free to suggest better alternatives if you can think of any.

I'll add tests once the code is reviewed.

[nirvn]

@nyalldawson , your comments have been addressed, and a test added.

[nyalldawson]

Nearly there!