Add SLD export support custom vector symbols and hatch fills - #7591

This pull request allows more MapServer vector styles to be accessed as SLD through WMS GetStyles requests. Styles in a Mapfile can then used by both MapServer, and by a client web application.

Background

SLD can be generated through MapServer URLs such as https://demo.mapserver.org/cgi-bin/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetStyles&layers=country_bounds which in this example returns the SLD of the country_bounds layer. This SLD can be read, for example (and for my main use case), by using the GeoStyler SLD Parser in a browser and applying it to a vector layer in OpenLayers using the GeoStyler OpenLayers parser.

MapServer currently supports exporting the 6 WellKnownName names in the core SLD specification - square, circle, triangle, star, cross, x. If these are used in a MapServer layer then the GetStyles request returns SLD such as:

<se:WellKnownName>circle</se:WellKnownName>

Updates

Hatches

This pull request adds support for hatch fills which are supported by GeoServer, and GeoStyler.

A Mapfile containing:

SYMBOL
	NAME "hatch"
	TYPE HATCH
END
	
LAYER
	NAME "polygon_snap_to_slash"
	TYPE POLYGON
	FEATURE
		POINTS
			0 0
			0 10
			10 10
			10 0
			0 0
		END
	END
	CLASS
		STYLE
			COLOR 0 0 255
			SYMBOL "hatch"
			ANGLE 30
			WIDTH 0.5
			SIZE 6 # this won't be output to the SLD
		END
	END
END

Will now produce the following SLD:

<se:Mark>
	<se:WellKnownName>shape://slash</se:WellKnownName>
	<se:Stroke>
		<se:SvgParameter name="stroke">#0000ff</se:SvgParameter>
		<se:SvgParameter name="stroke-width">0.5</se:SvgParameter>
	</se:Stroke>
</se:Mark>

The code will calculate the closest matching angle, so 80° will display as 90°, and output the following WellKnownNames:

• ~0° shape://horline
• ~45° shape://slash
• ~90° shape://vertline
• ~135° shape://backslash

Custom Vector Symbols

This PR also adds support for exporting any generic SYMBOL to SLD, by exporting it as an SVG, and then encoding it (to make it smaller in size), using GDAL's CPLBase64Encode function (GDAL is already a required MapServer dependency). Embedding the SVG in the SLD file avoids external markers and files, and issues around paths and security.

For example a Mapfile with the following:

SYMBOL
	NAME "pentagon"
	TYPE VECTOR
	FILLED TRUE
	POINTS
		0.5 0
		1 0.38
		0.81 1
		0.19 1
		0 0.38
		0.5 0
	END
END


LAYER
	NAME "pentagon"
	TYPE POINT
	FEATURE
		POINTS
			0 0
		END
	END
	CLASS
		STYLE
			SYMBOL "pentagon"
			COLOR "#156082"
			OUTLINECOLOR 255 0 0
			SIZE 16
		END
	END
END

Will produce a GetStyles response of:

<se:PointSymbolizer>
	<se:Graphic>
		<se:ExternalGraphic>
			<se:OnlineResource xlink:type="simple" xlink:href="data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxOCIgaGVpZ2h0PSIxOCIgdmlld0JveD0iMCAwIDE4IDE4Ij4KPHBhdGggZD0iTSA5LDEgTCAxNyw3LjA4IEwgMTMuOTYsMTcgTCA0LjA0LDE3IEwgMSw3LjA4IEwgOSwxIFoiIGZpbGw9IiMxNTYwODIiIHN0cm9rZT0iI2ZmMDAwMCIgc3Ryb2tlLXdpZHRoPSIxIi8+Cjwvc3ZnPgo="/>
			<se:Format>image/svg+xml</se:Format>
		</se:ExternalGraphic>
		<se:Size>16</se:Size>
	</se:Graphic>
</se:PointSymbolizer>

Ellipse symbols are also handled (by a separate function):

SYMBOL
	NAME "oval"
	TYPE ELLIPSE
	FILLED TRUE
	POINTS
		2 1
	END
END

Initially, I only added support for symbols which matched some of the GeoServer conventions such as shape://hexagon, but a generic SYMBOL to SVG function handles both "named" shapes and any other custom symbols, including
those with gaps using the -99 -99 breaks in the POINTS block. The older approach can be seen in earlier commits (e,g, 5173216).

Code and examples all written by me, with a review of the C++ by Claude.