perf(cmd/sointu-vsti): avoid reallocations of events array

Always appending to the end and consuming from the front cause the
capacity of the slice regularly running out, resulting in new
allocation. With this change, we increment index when consuming
events and append to the end, and when we reset, we move index to 0
and empty slice. This way, we always reuse the allocated memory.

cmd/sointu-vsti/main.go

@@ -16,14 +16,15 @@ import (
 )
 
 type VSTIProcessContext struct {
-	events []vst2.MIDIEvent
-	host   vst2.Host
+	events     []vst2.MIDIEvent
+	eventIndex int
+	host       vst2.Host
 }
 
 func (c *VSTIProcessContext) NextEvent() (event tracker.MIDINoteEvent, ok bool) {
-	var ev vst2.MIDIEvent
-	for len(c.events) > 0 {
-		ev, c.events = c.events[0], c.events[1:]
+	for c.eventIndex < len(c.events) {
+		ev := c.events[c.eventIndex]
+		c.eventIndex++
 		switch {
 		case ev.Data[0] >= 0x80 && ev.Data[0] < 0x90:
 			channel := ev.Data[0] - 0x80
@@ -66,7 +67,7 @@ func init() {
 		tracker := gioui.NewTracker(model, cmd.MainSynther)
 		tracker.SetInstrEnlarged(true) // start the vsti with the instrument editor enlarged
 		go tracker.Main()
-		context := VSTIProcessContext{make([]vst2.MIDIEvent, 100), h}
+		context := VSTIProcessContext{host: h}
 		buf := make(sointu.AudioBuffer, 1024)
 		return vst2.Plugin{
 				UniqueID:       PLUGIN_ID,
@@ -88,7 +89,8 @@ func init() {
 					for i := 0; i < out.Frames; i++ {
 						left[i], right[i] = buf[i][0], buf[i][1]
 					}
-					context.events = context.events[:0]
+					context.events = context.events[:0] // reset buffer, but keep the allocated memory
+					context.eventIndex = 0
 				},
 			}, vst2.Dispatcher{
 				CanDoFunc: func(pcds vst2.PluginCanDoString) vst2.CanDoResponse {