Spurious values in joystick reads

Every once in a while joystick x and y readings jump to 255.  It seems like it's every 5-10 seconds.

I added a check to simply ignore any reading where either is exactly 255.  We should either find a way to detect these (perhaps there's a pattern) or just filter them at the library level.  That would require caching the values and keeping old ones if the read was invalid (or just re-reading).