Return a MEOS struct as the struct, not as a pointer

[estebanzimanyi]

Three MEOS functions hand a struct back by value — tpoint_as_mvtgeom
answers MvtGeom, tgeo_space_split answers SpaceSplit,
tgeo_space_time_split answers SpaceTimeSplit — and a struct return is not
a pointer. On the ABI these bindings ship for, a 24-byte struct comes back
through a hidden first argument the caller supplies, so a binding that
declares the return as a pointer shifts every real argument by one register
and the callee reads the second argument where the first belongs. Measured on
tpoint_as_mvtgeom over a temporal point and an STBox: with the return
declared void * the call answers
The temporal value must be a temporal point, MEOS validating the STBox it
receives in place of the temporal value, and with the return declared
MvtGeom the same call answers count=2 and two live pointers.

The C# form of such a struct comes from the catalog's own structs entry:
field for field, a pointer as IntPtr and a scalar through the same map the
parameters use, under [StructLayout(LayoutKind.Sequential)]. Only a struct a
function actually returns or takes by value gets one, which the generator
reads off the signatures rather than a list — every other MEOS struct crosses
the boundary as a pointer and stays opaque.

Two tests read every field of one. The MvtGeom test reads its geometry back
through geo_as_text and walks its times array for exactly count
increasing timestamps; the SpaceSplit test reads each fragment through
tspatial_as_text and each bin through geo_as_text. A field at the wrong
offset, or a call whose arguments shifted, answers neither.