Change request done function arglist.

Specifically, change the first arg from an LX_OP * to an LX_CONN *.
This is mostly because of extensions, for whom LX_OP is opaque.  So is
LX_CONN, but it is the LX_CONN that the vast majority of uses in done
functions need.

This does cause a little trouble for conv-lookup-color.c; if I need a
wrapper like that for an extension, I'll have to figure out something
better.