[PostgreSQL provider] Retrieve CRS from spatial_ref_sys

[rouault]

instead of relying on QgsCoordinateReferenceSystem::createFromPostgisSrid()
which contrary to what its name suggest doesn't use postgis at all, but
the QGIS CRS database.

I detected the issue initially on a database where my spatial_ref_sys
table had been emptied and a few entries where assigned with SRIDs in
the EPSG range but with definitions that weren't the ones from EPSG.
There is no guarantee that EPSG:XXXX gets a SRID of XXXX

And the past implementation of the fallback case of QgsPostgresProvider::crs()
only looked at proj4text instead of using the WKT from srtext.

[nyalldawson]

Great, this has been on my wishlist for a long time!

Is the postgres id in QgsCoordinateReferenceSystem still used anywhere? Would be nice to deprecated that as it's quite dangerous/misleading API.

[rouault]

Is the postgres id in QgsCoordinateReferenceSystem still used anywhere?

createFromPostgisSrid() is no longer needed, and I've added a commit removing it (was private), but I see postgisSrid() is called in quite a few place. I guess in most of them it must be wrong, except the 2 places in the postgres provider where we might actually get a spatial_ref_sys SRID and should do that in another way. But I'd say cleanup for later...

[nyalldawson]

But I'd say cleanup for later.

Thanks, it's a good step forward!