Don't allocate a slice of windows 60 times per second

We were allocating and appending to a slice 60 times per second to modify the window list if windows were supposed to close. This updates the runloop to keep track of how many windows need to be closed and only allocate a slice when we need to.

Updates #2506

internal/driver/glfw/loop.go

@@ -139,24 +139,15 @@ func (d *gLDriver) runGL() {
 			}
 		case <-eventTick.C:
 			d.tryPollEvents()
-			newWindows := []fyne.Window{}
-			reassign := false
+			windowsToRemove := 0
 			for _, win := range d.windowList() {
 				w := win.(*window)
 				if w.viewport == nil {
 					continue
 				}
 
 				if w.viewport.ShouldClose() {
-					reassign = true
-					w.viewLock.Lock()
-					w.visible = false
-					v := w.viewport
-					w.viewLock.Unlock()
-
-					// remove window from window list
-					v.Destroy()
-					w.destroy(d)
+					windowsToRemove++
 					continue
 				}
 
@@ -177,20 +168,38 @@ func (d *gLDriver) runGL() {
 					}
 				}
 
-				newWindows = append(newWindows, win)
-
 				if drawOnMainThread {
 					d.drawSingleFrame()
 				}
 			}
-			if reassign {
+			if windowsToRemove > 0 {
+				oldWindows := d.windowList()
+				newWindows := make([]fyne.Window, 0, len(oldWindows)-windowsToRemove)
+
+				for _, win := range oldWindows {
+					w := win.(*window)
+					if w.viewport == nil {
+						continue
+					}
+
+					if w.viewport.ShouldClose() {
+						w.viewLock.Lock()
+						w.visible = false
+						v := w.viewport
+						w.viewLock.Unlock()
+
+						// remove window from window list
+						v.Destroy()
+						w.destroy(d)
+						continue
+					}
+
+					newWindows = append(newWindows, win)
+				}
+
 				d.windowLock.Lock()
 				d.windows = newWindows
 				d.windowLock.Unlock()
-
-				if len(newWindows) == 0 {
-					d.Quit()
-				}
 			}
 		}
 	}

internal/driver/glfw/window.go

@@ -284,7 +284,7 @@ func (w *window) destroy(d *gLDriver) {
 	w.DestroyEventQueue()
 	cache.CleanCanvas(w.canvas)
 
-	if w.master {
+	if w.master || len(w.driver.windowList()) == 1 {
 		d.Quit()
 	} else if runtime.GOOS == "darwin" {
 		go d.focusPreviousWindow()