asfdemux: fix performance issue, especially with high-bitrate streams

asfdemux: fix performance issue, especially with high-bitrate streams

Two things were suboptimal from a performance point of view:

a) consider a large media object such as a video keyframe, which
   may be split up into multiple fragments. We would assemble
   the media object as follows:
     buf = join (join (join (frag1, frag2), frag3), frag4)
   which causes many unnecessary memcpy()s, and malloc/free,
   which could easily add up to a multiple of the actual object
   size. To avoid this, we allocate a buffer of the size needed
   from the start and copy fragments into that directly.

b) for every fragment to join, we would create a sub-buffer
   before joining it (which would discard the sub-buffer again),
   leading to unnecessary miniobject create/free churn.

Conflicts:

	gst/asfdemux/asfpacket.c
	gst/asfdemux/asfpacket.h