Buffers for marker symbols

[nyalldawson]

This PR implements "buffers" for marker symbols. Just like label buffers, marker buffers are a halo effect around marker symbols designed to make the symbol more readable against different backgrounds.

Buffers are defined on a symbol level (not per symbol-layer), and are calculated and drawn using the shape of ALL the symbol layers in the marker. This makes them super-easy to configure, as getting the same result by hand would require many duplicate symbol layers and duplicate settings across these!

This change is designed to plug on of the (very few!) remaining large symbology gaps between ArcMap and QGIS, where ArcMap has had support for solid color marker "halos" for decades.

Sponsored by North Road, thanks to SLYR

Peek.2024-07-24.15-04.mp4

[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
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Thank you!

[qgis-bot]

@nyalldawson

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[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 19b780f)

[nyalldawson]

The test mask updates here relate to the clang tidy warning fix in f2663f1, not directly related to this PR...

[nyalldawson]

I'll merge this later today if there's no objections

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9191
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!