[feature] Add "Linear Referencing" symbol layer type

[nyalldawson]

This new symbol layer type allows placing text labels at regular intervals along a line (or at positions corresponding to existing vertices). Positions can be calculated using Cartesian distances, or interpolated from z/m values.

Functionality includes:

• Labels can be placed using fixed cartesian 2d distances, at regular linearly interpolated spacing calculated using the Z or M values in geometries, or at existing vertices
• Labels can show either the running total distance, or the linearly interpolated Z/M value
• Uses text renderer to draw labels, so the full range of functionality is available for the labels (including buffers, shadows, etc)
• Uses the QGIS numeric format classes to format numbers as strings, so users have full range of customisation options for eg decimal places
• An optional "skip multiples of" setting. If set, then labels which are a multiple of this value will be skipped over. This allows construction of complex referencing labels, eg where a symbol has two linear referencing symbol layers, one set to label every 100m in a small font, skipping multiples of 1000, and a second set to label every 1000m in a big bold font
• Labels are rendered using an angle calculated by averaging the linestring, so sharp tiny jaggies don't result in unslightly label rotation
• Optionally, markers can be placed at referenced points in the line string, using a full QGIS marker symbol (this allows eg showing a cross-hatch at the labeled point, for a "ruler" style line)
• Data defined control over the placement intervals, skip multiples setting, marker visibility and average angle calculation length

Notes:

• When using the distance-based placement or labels, the distances are calculated using 2D only, Cartesian calculations based on the original layer CRS. This could potentially be extended in future to expose options for 3D Cartesian distances, or ellipsoidal distance calculations.

Sponsored by the Swiss QGIS User Group

[qgis-bot]

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

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[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 f7930bd)

[nirvn]

Purely cosmetic: why not using the same style for mNumericFormat here? I.e.

Suggested change

	res->mMarkerSymbol.reset( mMarkerSymbol ? mMarkerSymbol->clone() : nullptr );
	if ( mNumericFormat )
	res->mNumericFormat.reset( mNumericFormat->clone() );
	res->mMarkerSymbol.reset( mMarkerSymbol ? mMarkerSymbol->clone() : nullptr );
	res->mNumericFormat.reset( mNumericFormat ? mNumericFormat->clone() : nullptr );

[nyalldawson]

Cos the marker symbol is optional, whereas we expected that the numeric format should always be set. I'm playing things extra careful with the if ( mNumericFormat ) check, but if somehow that happens then at least we won't overwrite the default format created for the cloned copy...

[nyalldawson]

Thanks @nirvn, all addressed!

[qgis-bot]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#9231
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!