Set midi messages timestamp relative to buffer start

[DamRsn]

ExternalPlugin: Fix MIDI Message Timestamps

Problem

In external plugins, MIDI messages are timestamped relative to the start of the full audio sequence, causing issues when handling note_on or note_off messages with timestamps greater than the bufferSize. This results in notes not being played or not stopped as expected.

Solution

To resolve this issue, I modified the renderMIDIMessages() function in the ExternalPlugin module. The sampleDeltaToAdd argument in midiChunk.addEvents() is changed from 0 to -i, ensuring that each MIDI event is timestamped according to the start of the current buffer and not the entire sequence.

Result

After this change, MIDI messages will have timestamps relative to the start of the current processBlock buffer, allowing external plugins to handle them correctly.

This change addresses the issue reported in #258