indxf: prescan all objects and types

[rurban]

to enable prettier ref -> TYPE (name) logging,
and also to calloc the dwg->objects array earlier, and get away with much less dirty_refs,
which helps the importers greatly.

also doable for decode, not just the importers. with decode just scan the HANDLES (object-map) for its types.
for indxf just scan all 0 lines.