basetransform: improve prepare_output_buffer

basetransform: improve prepare_output_buffer

Clean up the prepare_output_buffer function.
Reuse the input buffer when it is writable and when doing an
in-place but non-passthrough transform.
Move the copy-metadata function call to the prepare_output_buffer default
function. If subclasses implement a custom prepare_output_buffer, they must also
copy the metadata themselves.
Remove a useless memory copy because prepare_output_buffer already did that.