explictly fail to draw unpremul, as this is buggy, incomplete, tricky, and unused

BUG=skia:

Change-Id: Ie9da6015cc4955c0f27f6db53fc6ae532e0132f4
Reviewed-on: https://skia-review.googlesource.com/7304
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
4 files changed