evdev: Store positions for all touch points in the evdev device

evdev: Store positions for all touch points in the evdev device

We need to store all touchpoint positions so that if we just get an
ABS_MT_POSITION_X or Y event, we can pull the other coordinate from the
cache.  And we need this across invocations of evdev_input_device_data(),
so the accumulator approach doesn't work.

Instead, we go back to the approach of storing all this state in the
evdev device struct and we might as well just move the rel and abs state
there too.