FrameSvgPrivate::generateBackground: generate background also if reqp != p

Summary:
d8a1a9eb084b19e552c789244267f7346e1b27a8 changed the condition from
hasElementPrefix(q->prefix()) to hasElementPrefix(frame->requestedPrefix)
but it is not clear why, as the background is generated from the prefix
data & the requestedPrefix is no-where in effect.
As a result sometimes the background is not rendered for a given
not-existing requestedPrefix which was mapped to an empty prefix instead.

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21175