Fixed Canvas ImageData pixel values not being settable to 0.

Someone probably figured "since the data is all initialized to 0 to
begin with, we can skip 0 values". However, it's possible to temporarily
set a value to other than 0 and then back to 0, a fully valid use case
that we need to support.

Task-number: QTBUG-29065
Change-Id: Ia9f0803743d696ca8b9cca89c666ccba80a3abd0
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>