toytoolkit: rename tile_mask to render_shadow

This function is used and clearly designed only for drawing the shadows.
Rename it so that it has name after what it does and also move some
common code into the function.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>