Stopped adding a grain of sand per step.
Collapsing should work properly for any amount of sand grains,
regardless of whether it's 4 or 4000. Previous code fixed it at 4
always, and how it distributed was so as well. Now we distribute as
much sand as we can.
This comes from the idea that, instead of incrementing while
rendering, let's just set some n pixels in the centre and see how the
model distributes them.
