Previously, if a state buffer was needed and the existing one did not
match the size and type exactly, the old buffer was freed and a new
buffer was malloced. However, malloc can be slow and it is possible to
reuse a buffer if the new one being asked for is smaller than the old
one. Thus, change the semantics to use a previously allocated buffer if
it is at least as big as needed. This might leave pools of data larger
than what we need, but will reduce the overall number of malloc calls
needed.