til: introduce some generic fragmenters for convenience

Most of the threaded modules have settled down on two basic forms
of fragmenter function:

1. a slice per cpu, where tile-oriented locality isn't useful
2. ~64x64 tiles, in scenarios where screen-space locality helps

Now that n_cpus is wired up to the fragmenter, #1 can be
fulfilled without requiring a module-private context plumbing
n_cpus from create_context().

A future commit will replace some module-specific fragmenters by
returning one of these instead as res_fragmenter in their
prepare_frame(), wherever applicable.

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