Fix MIDI port offsets.

Fix MIDI port offsets.

AudioPort::get_audio_buffer() can offset the buffer simply by offsetting
a pointer. This allows to get an offset buffer for a given port.

For MIDI there's no such concept. A method writing to a MIDI buffer
which is backed by a Port can at best offset it by the global port-buffer
offset (static Port::port_offset), but not by the individual target port's
offset.