MVT test failure in trunk

[dbaston]

An MVT test failure is caused by r16671, as discussed in ​https://github.com/postgis/postgis/pull/268#issuecomment-407780730

One solution is described by Raul in the linked discussion. Another option is the following:

1) Update lwgeom_subdivide to use lwgeom_intersection instead of lwgeom_clip_by_rect.

2) Revert lwgeom_clip_by_rect to its 2.4 state (changes are based on a bug report that cannot be confirmed)

This should make MVT clipping work as it did in 2.4. I verified that it works locally but it requires rewriting the MVT tests to use ST_Equals instead of text comparison.

[Algunenano]

I'll add that ST_AsMVTGeom was ok with invalid geometries, either as input or after some of its internal transformations. Part of that was lwgeom_clip_by_rect being ok with them too. And, at least for 2.5, we want to keep this behaviour. I'll do some performance tests of the possible solutions.

This should make MVT clipping work as it did in 2.4. I verified that it works locally but it requires rewriting the MVT tests to use ST_Equals instead of text comparison.

I'm avoiding St_Equals as it ignores directionality, which is mandatory in the MVT spec 2.1, and we have already been bitten by it (the tests used to ignore it, so introducing bugs was pretty easy).

[dbaston]

Understood about directionality. The differences I was seeing were in the order of components and start point:

-PG9|POLYGON((4096 4096,0 4096,0 0,4096 0,4096 4096))
+PG9|POLYGON((0 4096,0 0,4096 0,4096 4096,0 4096))

-PG43 - ON |MULTIPOLYGON(((0 10,5 5,10 10,0 10)),((5 5,0 0,10 0,5 5)))
+PG43 - ON |MULTIPOLYGON(((5 5,0 0,10 0,5 5)),((0 10,5 5,10 10,0 10)))

[dbaston]

In 16679:

Revert lwgeom_clip_by_rect to use GEOSClipByRect

This commit reverts lwgeom_clip_by_rect to its implementation in PostGIS 2.2 -
2.4.

It makes trivial modifications to the MVT tests to reflect the different results
returned by GEOSClipByRect instead of GEOSIntersection. The changed results are
topologically equivalent to the previous results, and their orientation is
unchanged.

No tests have been commented out or made version-dependent.

Closes #4134
Closes #4135
References #4038
Closes ​https://github.com/postgis/postgis/pull/282