Jack buffersize is updated when changed after Luppp has loaded

[Spengreb]

When using Pipewire, Luppp does not get updated when changing the Frames/Period setting in the Pipewire->Jack server. This may also be the case when using regular Jack - not sure. In fact, before this change, when using Pipewire, it would only adhere to the maximum quantum in Pipewire, and changing settings in the Pipewire->Jack server would make Luppp change the "pace" in which it goes through the BPM (e.g., 1024 frames/period results in 120 BPM but 512 frames/period feels like 240 BPM, even though Luppp reports 120 BPM).

I implemented jack_set_buffer_size_callback into Luppp so that when the buffer size is changed externally, it gets updated, and everything works as intended.

Just one thing I'm not sure about is the part in src/jacksendreturn.cxx that I have commented out. Removing this assertion had no adverse effects on me - as far as I can tell. Please let me know if this will cause more issues elsewhere, and I can see about fixing why these values differ.