Incorrect geometry bbox returned from ST_SnapToGrid

[Algunenano]

Following the #4493 and #4494 drill:

WITH geom AS
(
    SELECT ST_SnapToGrid('POLYGON((0 0, 10 0, 10 10, 10.6 10, 10.5 10.5, 10 10, 0 10, 0 0))', 10) as g
)
Select ST_AsText(g) as geometry, postgis_getbbox(g) AS box from geom;
              geometry              |              box               
------------------------------------+--------------------------------
 POLYGON((0 0,10 0,10 10,0 10,0 0)) | BOX(0 0,10.6000003814697 10.5)
(1 row)

[Algunenano]

By following the /* COMPUTE_BBOX TAINTING */ comment I see this affects LWGEOM_SetEffectiveArea, LWGEOM_ChaikinSmoothing and LWGEOM_snaptogrid, LWGEOM_snaptogrid_pointoff.

The fix is simple, but I'll try to come up with test to along with it for each function.

[Algunenano]

After testing it, the only affected functions are LWGEOM_snaptogrid and LWGEOM_snaptogrid_pointoff, since LWGEOM_SetEffectiveArea and LWGEOM_ChaikinSmoothing don't reuse the input and the output doesn't have a bbox (so it's calculated with lwgeom_add_bbox).

I've added tests for all 4 functions and the fix for the 2 affected by the issue in ​https://github.com/postgis/postgis/pull/473

[Raul Marin]

In 17798:

Fix ST_SnapToGrid output having an outdated bbox

References #4495
Closes ​https://github.com/postgis/postgis/pull/473

[Raul Marin]

In 17799:

Fix ST_SnapToGrid output having an outdated bbox

References #4495

[Raul Marin]

In 17800:

Fix ST_SnapToGrid output having an outdated bbox

References #4495

[Raul Marin]

In 17801:

Fix ST_SnapToGrid output having an outdated bbox

Closes #4495