Clean up Transform module interface; Add set_dst_geo_transform

set_dst_geo_transform is required for proper warping
WIP attempt to fix warp segfaults

warptut.native segfaults while warptut.byte
does not (Ubuntu 12.04 64bit,
OCaml 4.01.0 and 4.02.0).  The issue seems to
be something being GC'd or moved during a
callback.