Make sure pictures draw Clears even when the clip is empty.

We fix this by rewriting empty queries to very tiny queries, which will certainly
hit ops that span the entire picture (like Clear) and hopefully not much more.

(This doesn't quite work in the full cull rect world if [0,0,ε,ε] doesn't
overlap the picture.  Let's cross that bridge when we get there.)

BUG=432991

Review URL: https://codereview.chromium.org/732723004
2 files changed