Preventing division by 0 in non-separable blend mode shaders.

In the software path, the same issue has been fixed some time ago:
https://codereview.chromium.org/114173002

BUG=skia:

Review URL: https://codereview.chromium.org/666043003
dstCopy on optdrawstate

BUG=skia:

Review URL: https://codereview.chromium.org/735363003
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
Stop overloading internal_dispose in SkTextBlob and SkData

Review URL: https://codereview.chromium.org/737093002