CoreMIDI: Fix time delta for messages within same MIDI packet

[markovicpp]

Single MIDI packet can contain multiple MIDI messages.
The first message within the packet has time delta relative to previous MIDI packet, but remaining MIDI messages have time delta 0 relative to the first message.

[markovicpp]

Using cmidiin test as a listener on virtual port, then sending 2 note ons (64, 68) followed by 2 note offs after a second could be delivered in 2 MIDI packets, each having 2 messages.

Byte 0 = 144, Byte 1 = 64, Byte 2 = 127, stamp = 0
Byte 0 = 144, Byte 1 = 68, Byte 2 = 127, stamp = 0

Byte 0 = 128, Byte 1 = 64, Byte 2 = 127, stamp = 1.00275
Byte 0 = 128, Byte 1 = 68, Byte 2 = 127, stamp = 1.00275

The last stamp is incorrect and should be 0.