Canvas: do not emit paint when resized but hidden

It turns out to be a bad idea (as in, breaks the enterprise controls)
to block paint() altogether when hidden. The original bug report
QTBUG-31830 only complained about paint() being emitted upon resize
while _hidden_, so this change solves exactly that. User is still
free to call requestPaint() even when hidden, and Canvas now doesn't
recklessly ignore the request.

Task-number: QTBUG-31830
Change-Id: Icd82188f65406b3ef6ae3b19b5984944576886ff
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>