Oh, right. I don't think we want to clear before every draw, no. There may be an argument for clearing at the start of each test, although I think that in that case we'd like to clear with something other than zeroes.
Agreed. I wouldn't even bother clearing with something different than zero. Just clear with zero, and if the tests needs something else they can clear explicitly (or render with a trivial pixel shader, if we don't want a `clear` command).