Some MIDI events(ex. sustain pedal) should be ignored while exporting

[PhysSong]

To reproduce:

1. Enable MIDI input in instrument window.
2. Put some notes.
3. Hold sustain pedal.
4. Export project.

Result: sustain pedal affects to the export, which is undesirable.
Tested with latest master.

[zonkmachine]

You don't need to export the project. Swap step 4 above with, 4. Play project.
This behaviour is already there in 1.0.3

[PhysSong]

Even more crazy thing: If you press keys from your MIDI keyboard while exporting, they will be rendered.

[Umcaruje]

Tbh all midi events should be completely ignored while exporting. I had to re-render a lot of times when I actually accidentally pressed some keys on my midi keyboard while exporting

[zonkmachine]

So, something like this...

void InstrumentTrack::processInEvent( ... )
{
    if( Engine::getSong()->isExporting() )
    {
        return;
    }
...

Edit: Code above works but doesn't cover notes/events in effect before exporting.

[zonkmachine]

I pulled the above code in #3778. It's a partial fix and a good compromise for 1.2.0 .