avoid allocation on every drawSingleFrame

internal/driver/glfw/loop.go

@@ -67,8 +67,15 @@ func runOnDraw(w *window, f func()) {
 	<-done
 }
 
+// Preallocate to avoid allocations on every drawSingleFrame.
+// Note that the capacity of this slice can only grow,
+// but its length will never be longer than the total number of
+// window canvases that are dirty on a single frame.
+// So its memory impact should be negligible and does not
+// need periodic shrinking.
+var refreshingCanvases = make([]fyne.Canvas, 0)
+
 func (d *gLDriver) drawSingleFrame() {
-	refreshingCanvases := make([]fyne.Canvas, 0)
 	for _, win := range d.windowList() {
 		w := win.(*window)
 		w.viewLock.RLock()
@@ -89,6 +96,12 @@ func (d *gLDriver) drawSingleFrame() {
 		refreshingCanvases = append(refreshingCanvases, canvas)
 	}
 	cache.CleanCanvases(refreshingCanvases)
+
+	// cleanup refreshingCanvases slice
+	for i := 0; i < len(refreshingCanvases); i++ {
+		refreshingCanvases[i] = nil
+	}
+	refreshingCanvases = refreshingCanvases[:0]
 }
 
 func (d *gLDriver) runGL() {