This is fun, but it really needs the ability to hold click when placing blocks.
I was going to say "increased brush size" (by the time I'm buying 70 pixels at once, I wouldn't mind painting in a 2x2 area), but no, this is absolutely what it needs.
(I also found it slowed down above some point, which meant that I didn't end up filling the whole screen. Which is a shame; I wanted to see what it looked like.)