Add lookups and database functions to check for valid geometries and repairing them.

[Daniel Wiesmann]

There are many data sources that come with geometries that are not valid, and clipping or intersecting functions may lead to invalid geometries sometimes. At the same time, geographic operations often require valid input.

The python geometries in GeoDjango have a valid property, but the same property is not available during database queries.

To avoid errors during geoprocessing on large datasets the database side, it can be essential to be able to filter valid geometries and possibly repair them as well.

[Daniel Wiesmann]

Here is the pr: ​https://github.com/django/django/pull/6412

I tried to find equivalent functions in the other spatial backends, the only function I found was SDO_GEOM.VALIDATE_GEOMETRY_WITH_CONTEXT. The result of this function is an error code for invalid geometries. Unfortunately I am not familiar with oracle so I dont know how to use this properly.

​https://docs.oracle.com/cd/B28359_01/appdev.111/b28400/sdo_objgeom.htm#BGHFDDBF

[Tim Graham <timograham@…>]

In c12a00e:

Fixed #26455 -- Allowed filtering and repairing invalid geometries.

Added the IsValid and MakeValid database functions, and the isvalid lookup,
all for PostGIS.

Thanks Tim Graham for the review.

[Tim Graham <timograham@…>]

In 65006e0b:

Refs #26455 -- Documented isvalid/makevalid in compatibility table.