Fix bug that ignores tile size and tile_motion.

In backend.py, (line 66) I forgot to multiply `tile_motion` (fraction of tile
required to register motion) by `tile_area`. This meant that even when
`tile_motion` was set to 1 (100%) and tile_width = tile_heigth = 20,
a single pixel of motion would register as "motion".

However, it should have worked like this:
tile_area = tile_width * tile_height = 400 pixels.
tile_motion = 1.
Motion will not register unless ALL 400 pixels exceed the motion
threshold for that particular tile.

The bug is now fixed, and tests have been added to check for this.
From now on, this should work.