delta timestamps are broken on macOS in current release (1.5.8)

[dklghub]

on macOS, when rtmidi receives midi packages containing multiple midi messages, the callback is called multiple times with the same delta time value. this was fixed a couple of months ago in rtmidi here: https://github.com/thestk/rtmidi/commit/eafe2f72a6492e7156c6a4f2771eb7ceb264e97b

this is a major issue for code that requires accurate timing so a new release might be motivated.