basetransform: Add buffer list support

Submitted by Sebastian Dröge @slomo

Link to original bug (#746544)

Description

Currently basetransform has no buffer list support at all. At least for elements that don't do much, like capsfilter and identity, this would be really useful to have. OTOH for elements that do actual processing, it can reduce latency to split the buffer list into separate buffers and even can make the difference between buffers arriving in time or too late inside the sink.

I would propose to add a transform_list() and transform_list_ip() vfunc to basetransform for this, and let the default do what is happening right now already. And inside identity and capsfilter it just passes through the list.

Thoughts?