Reinstate sfcgal functions

[lbartoletti]

In version 2.x of PostGIS, there was a mechanism for choosing the backend between
GEOS and SFCGAL for certain operations. When adding the SFCGAL extension, the
backend was removed, and some SFCGAL functions that duplicated GEOS functionality
were also removed. This commit reintroduces these functions but prefixes them
with CG_ to distinguish them from the "classic" GEOS functions using the prefix
ST_.
Often, the 2D functions will produce the same result, sometimes with small
differences due to the algorithms or calculation methods used.

Fixes https://trac.osgeo.org/postgis/ticket/5405

[lbartoletti]

For the documentation, I added each function in a new sub-folder to make the file reference_sfcgal.xml more digestible. It seems to me that we have to add the ENTRY in postgis.xml; I couldn't do it in reference_sfcgal.xml.
In another PR, I will put the functions prefixed with CG_ instead of ST_ in the same sub-folder, and I will update and organize the functions in reference_sfcgal.xml.

[robe2]

For the documentation, I added each function in a new sub-folder to make the file reference_sfcgal.xml more digestible. It seems to me that we have to add the ENTRY in postgis.xml; I couldn't do it in reference_sfcgal.xml. In another PR, I will put the functions prefixed with CG_ instead of ST_ in the same sub-folder, and I will update and organize the functions in reference_sfcgal.xml.

I'd rather we not break each function into a separate file. It's not like there are that many that reference_sfcgal.xml is that bloated. It would also break convention, as other extensions and even postgis proper, do not have a separate file for each function.

We could have more than one sfcgal file, if you feel there are distinct themes deserving their own dedicated file.

[lbartoletti]

For the documentation, I added each function in a new sub-folder to make the file reference_sfcgal.xml more digestible. It seems to me that we have to add the ENTRY in postgis.xml; I couldn't do it in reference_sfcgal.xml. In another PR, I will put the functions prefixed with CG_ instead of ST_ in the same sub-folder, and I will update and organize the functions in reference_sfcgal.xml.

I'd rather we not break each function into a separate file. It's not like there are that many that reference_sfcgal.xml is that bloated. It would also break convention, as other extensions and even postgis proper, do not have a separate file for each function.

We could have more than one sfcgal file, if you feel there are distinct themes deserving their own dedicated file.

OK, done in a6dac83

[robe2]

@lbartoletti Now that you have commit rights, feel free to commit this yourself. Let me know if you have any questions.

[robe2]

All looks good to me

[lbartoletti]

@lbartoletti Now that you have commit rights, feel free to commit this yourself. Let me know if you have any questions.

Yes, I didn't see anything in the documentation. But, is it safe to merge on github, or should I push my commits to gitea?

I opened here, to check the Cirrus CI at the same time. I've seen other PRs integrated from Github, but I'm intrigued by this sentence in the documentation:
Changes committed to mirror sites will be overwritten the next time a change is synchronized from OSGeo.

[lbartoletti]

Pushed in https://git.osgeo.org/gitea/postgis/postgis/commit/3b516cba062739f0f7992827d57217152948d155