Make EffectFrames work with Nouveau driver.

This worksaround a problem with the nouveau driver causing the text frames to be incorrectly rendered. We need to keep the QPixmap around as long as we have a texture created from that texture.

This applies for the text and the unstyled effect frame. For the frames generated from Plasma's FrameSvgs it is not required.
Addresses freedesktop.org bug 30286.