videodecoder: getters for pool and allocator

videodecoder: getters for pool and allocator

Sometimes the decoder would need to use the pool or the allocator for
something else than just allocating output buffers. For example, the querying
for different parameters, such as asking for a bigger number of buffers to
allocate in the pool.

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