buffer: unify buffer merge methods
Add gst_buffer_append() which appends the memory blocks from one buffer to another. Remove the old inefficient _merge() and _join() methods which forced a premature memcpy in most cases. Remove the _is_span() and _span() methods they are not needed anymore now that we can _append(). Merging and spanning will be delayed until mapping or maybe not at all when the element can deal with the different memory blocks.
Showing with 64 additions and 252 deletions