Add SLD 1.0 export for rasters

[luipir]

Description

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
• I have run the scripts/prepare-commit.sh script before each commit

[luipir]

I'll check failing spell check tomorrow... I do not understand why prepare_commit.sh hasn't been applied

[nyalldawson]

So the desire is to merge in 3.6 and then backport to 3.4?

[luipir]

So the desire is to merge in 3.6 and then backport to 3.4?

right, was my error writing the email in dev list

and thanks for the review... I'll work on it

[luipir]

@nyalldawson and @jef-n let me know if you need more to review

[nyalldawson]

Looks good to me. I can't review the actual SLD logic, but the code looks good.

[luipir]

I can't review the actual SLD logic

@nyalldawson I tested against a geoserver 2.14 version. I suggest to leave to @aaime the last word about SLD correctness.

[aaime]

I had a look at the code and tested interactively the various export modes. Looks good to me, +1