vtdec: handle non-consecutive GstBuffer input without copying

vtdec: handle non-consecutive GstBuffer input without copying

CMBlockBuffer offers a model similar to GstBuffer, as it can
consist of multiple non-consecutive memory blocks.

Prior to this change, what we were doing was:

 1) Incorrect:

   CMBlockBufferCreateWithMemoryBlock does not copy the data,
   but we gst_buffer_unmap'd right away.

 2) Inefficient:

   If the GstBuffer consisted of non-contiguous memory blocks,
   gst_buffer_map resulted in malloc / memcpy.

With this change, we construct a CMBlockBuffer out of individual mapped
GstMemory objects. CMBlockBuffer is made to retain the GstMemory
objects (through the use of CMBlockBufferCustomBlockSource), so the
original GstBuffer can be unref'd.

https://bugzilla.gnome.org/show_bug.cgi?id=751241