Make refresh on viewport no-op if there's nothing to do

[fstanis]

Looking at the code for refresh in viewport, it seems it will call self._device.display even when there's nothing to do:

luma.core/luma/core/virtual.py

Lines 113 to 127 in 6b69584

	def refresh(self):
	should_wait = False
	for hotspot, xy in self._hotspots:
	if hotspot.should_redraw() and self.is_overlapping_viewport(hotspot, xy):
	pool.add_task(hotspot.paste_into, self._backing_image, xy)
	should_wait = True
	if should_wait:
	pool.wait_completion()
	im = self._backing_image.crop(box=self._crop_box())
	if self._dither:
	im = im.convert(self._device.mode)
	self._device.display(im)

Since this function is usually called often (i.e. once per frame), would it make sense to skip this step if the image hasn't changed (i.e. should_wait == false)? I understand refresh is also called from display and set_position in which case it wouldn't be desirable to skip this step, but this could be mitigated by adding a flat to it, i.e. refresh(force = False) and calling it as refresh(True) from those two spots.