As promised in several previous patches, this is a mechanical change
that does two things to every filter: instead of using void* for
callback parameters, we now use an actual opaque type (which helps
avoid errors of passing the wrong pointer around), and instead of
passing identical pointers for next_ops and nxdata, we pass a single
parameter.

The changes in docs and server are manual, but the bulk of the changes
in filters were driven by:

$ cat s.cocci
@rule1@
identifier fn, next_ops, nxdata;
typedef nbdkit_next;
@@
 fn(
- struct nbdkit_next_ops *next_ops, void *nxdata
+ nbdkit_next *next
 ,...)
 {
 ...
 }

@@
identifier rule1.next_ops, rule1.nxdata;
identifier op;
expression list args;
expression exp;
@@
(
-next_ops->op (nxdata, args)
+next->op (next, args)
|
-exp (next_ops, nxdata, args)
+exp (next, args)
)

@@
identifier fn, next, nxdata;
@@
-fn(struct nbdkit_next_open *next, void *nxdata
+fn(struct nbdkit_next_open *next, nbdkit_context *nxdata
 ,...)
 {
 ...
 }

@@
identifier fn, next;
identifier nxdata != handle;
type nbdkit_foo;
@@
 fn(
- nbdkit_foo *next, void *nxdata
+ nbdkit_foo *next, nbdkit_backend *nxdata
 , ...)
 {
 ...
 }
$ spatch --sp-file s.cocci --in-place --no-show-diff \
  filters/ tests/test-layers-filter.c
$ git grep -l 'next->[a-z_]*(' filters/ | \
  xargs sed -i 's/next->\([a-z_]*\)(/next->\1 (/g'

Followed by manual touchups until everything compiled.