ST_AsMVTGeom: Uniformize multigeometry return types

[Algunenano]

Depending on the result of lwgeom_make_valid, ST_AsMVTGeom might return a single geometry (polygon or line) or a multigeometry (multipolygon or multiline) formed by only 1 geometry.

I plan to modify this behaviour for the sake of consistency. Since it's really hard to determine if the input was a multipolygon / polygon once you accept geometry collections, I'm going to choose to return POLYGON(geometry) in all cases where the final result is a MULTIPOLYGON((single geometry)), and the equivalent for MULTILINE.

This change only affects 2.5 and trunk as 2.4 had a different behaviour.

[algunenano]

In 17160:

ST_AsMVTGeom: Always return the simplest geometry

References #4300

[algunenano]

In 17161:

ST_AsMVTGeom: Always return the simplest geometry

Closes #4300

[algunenano]

In 17169:

ST_AsMVTGeom: Transform coordinates space before clipping

References #4300

[algunenano]

In 17170:

ST_AsMVTGeom: Transform coordinates space before clipping

References #4300