Constify struct filter access and introduce filter_get().

Constify struct filter access and introduce filter_get().

This modifies all users of the filter API to not access the filter
array directly. Instead all callers now obtain a const pointer to
the filter structure through the new filter_get() accessor function.

However, the filter array can not be made constant yet because the
->init methods modify the filter structure. This requires some casts
in filter_common.c unfortunately.