Feature: function to force orientation of polygon rings

[jorisvandenbossche]

Being able to force the orientation of rings to be (counter-)clock-wise would be useful for certain use cases (eg some file formats or applications require a specific orientation).

For reference, PostGIS has this functionality as ST_ForcePolygonCCW and ST_ForcePolygonCW.
Shapely has this functionality as shapely.ops.orient(..) based on a custom python implementation (impl). But, this is currently a pure python function and not available as fast vectorized function (wrapping GEOS at the C level), and so shapely could benefit from a GEOS C API function for this (shapely/shapely#1366).

GEOS already exposes a method to check for the orientation of a coordinate sequence in the C API function: GEOSCoordSeq_isCCW.

[jorisvandenbossche]

Some thoughts on a possible API: although GEOSCoordSeq_isCCW as prior art works on an individual coordinate sequence, for usability (eg in shapely), it would be easier to have a function that works on an actual GEOSGeometry and transforms any ring contained in the geometry (like the PostGIS functions).
Of course, something like this needs more logic (for example, the expectation is then to transform inner rings of polygons using the opposite orientation), and it is possible to keep this logic in consumers of the API like shapely.

On the other hand, if this logic lives in bindings like shapely, then you can already do that right now with GEOSCoordSeq_isCCW and GEOSReverse on the individual LinearRings.

Quoting @dr-jts from the shapely issue:

Perhaps a function GEOSOrientPolygon(Geometry, boolean isCW), to allow orienting either shell-CW (the GEOS default) or shell-CCW.