internal/driver/glfw: clean the cache periodically.

The cache is supposed to be cleaned periodically, but the glfw
driver does not do this as the mobile driver does.

This commit adds similar logic to the glfw driver as exists in
the mobile driver, to periodically clean the cache on paint events.

Fixes fyne-io#4903

internal/driver/glfw/loop.go

@@ -212,7 +212,7 @@ func (d *gLDriver) repaintWindow(w *window) {
 			w.shouldExpand = true
 			w.viewLock.Unlock()
 		}
-		canvas.FreeDirtyTextures()
+		refresh := canvas.FreeDirtyTextures() > 0
 
 		updateGLContext(w)
 		canvas.paint(canvas.Size())
@@ -225,6 +225,7 @@ func (d *gLDriver) repaintWindow(w *window) {
 		if view != nil && visible {
 			view.SwapBuffers()
 		}
+		cache.Clean(refresh)
 	})
 }