pipewiresink: use all the available dest memory when copying buffer

When pipewiresink needs to copy data, it has to resize the destination
buffer (to a smaller size) in order to send the correct data size to
pipewire. When this dest buffer is reused later, it will still have
this smaller size as its total size and the copy may discard data
from upstream if the new upstream buffer is bigger than the last one
that was copied on the same dest buffer.