Backport of Add SLD 1.0 export for rasters

[luipir]

Description

Backport of #9049
The PR add the ability to export QGIS raster style as SLD 1.0 with geoserver flavours

The PR has been sponsored by Geosolution [1]

[1] https://www.geo-solutions.it/

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and contain sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
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[jodygarnett]

GeoServer PSC is interested in seeing this backport made available to our GeoServer users (we gathered together several funding sources and a few folks did personal contributions to make it happen).

Reference:

• exporting-raster-symbology (GeoServer User Manual)

[nyalldawson]

I doubt you'll get anyone explicitly approve this backport and take responsibility here.... But... It's been a week with no objections. That usually indicates it's ok to proceed.

[luipir]

@nyalldawson I was in doubt how to read this silence

[luipir]

@jodygarnett you can update https://docs.geoserver.org/latest/en/user/styling/qgis/index.html#exporting-raster-symbology with 3.4 LTR

[jodygarnett]

How should I word that? Should we not wait until this PR shows up in an update?

[luipir]

sure... just put it in the todo list

[jodygarnett]

Okay checked the roadmap, so this will be in 3.4.5 due out shortly ... I can use that so it will be "correct". Done...