Needs #637.

• Makes renderer.pixel_ratio always have a value. Previously it can always be None. Now the default case is simply handled in the setter.
• Adds renderer.pixel_filter to control filtering in the flusher.
  • Can e.g. be set to higher value for a more blurry image.
  • Or to zero as requested in How to disable antialiasing in rendered image? #640.
  • Or set to zero and use a pixel_ratio of 0.125 to get huge pixels to e.g. work on line aa :)