return const OGRSpatialReference* for a few methods(fixes #7377)

[rouault]

• Make OGRCoordinateTransformation::GetSourceCS() and GetTargetCS() return a const OGRSpatialReference* (fixes Make OGRCoordinateTransformation::GetSourceCS/GetTargetCS() return a const OGRSpatialReference* #7377)

  to reflect that those objects should be considered immutable.

  The downside is that we have internally to
  const_cast<OGRSpatialReference*>, in particular in
  OGRGeometry::transform() when assigning GetTargetCS() to the geometry,
  as assignSpatialReference() needs to increase the reference count.

• Make OGRGeometry::getSpatialReference() return a const OGRSpatialReference*

• Make OGRGeomFieldDefn::GetSpatialRef() return a const OGRSpatialReference*

[elpaso]

LGTM, for the const_cast, would it be possible to mark Reference mutable and avoid the cast?

[rouault]

would it be possible to mark Reference mutable and avoid the cast?

I considered this. The issue is that we have a GetRefCount() method, so nRefCount is not just an implementation detail and Reference() has thus visible effects which make it a bit hard to tag is a const.