basetransform: getters for pool and allocator

basetransform: getters for pool and allocator

Sometimes a transform filter would need the buffer pool or the memory
allocator negotiated by the base class, for example, for querying different
parameters, such as a bigger number of buffers to allocate by the buffer pool.

This patch expose a two getters accessors: one for the buffer pool and the
other for the memory allocator.