Provide a function to read an LWGEOM bounding box and stop reading it directly

[strk]

In order to simplify the code, we'd need a function that returns an LWGEOM's bounding box optionally caching it in the structure if not available.

I belive computing the BBOX of a multigeometry by combining the rounded BBOX of component geometries should yeld the same numbers as computing everything using doubles and rounding only at the end, although the operation would be slightly less performance (could be impossible to note even by a performance analisys tool).

Such new function could replace the public compute_bbox functions (there are 3 of them: wrapper, geodetic, cartesian) which could then be moved to the internal header.

[strk]

An open question is if such method should also take care of rounding the BBOX to floats or not. Such rounding would serve the purpose of always having the _same_ BBOX for the _same_ geometry, no matter if the BBOX was read from the serialized format (which uses floats) or computed from the actual coordinates.