[FrameSvg] Use QPixmap::mask() instead of deprecated convoluted way via…

Authored by broulik on Aug 30 2017, 10:05 AM.

Description

[FrameSvg] Use QPixmap::mask() instead of deprecated convoluted way via alphaChannel()

QPixmap::alphaChannel() is deprecated.

Differential Revision: https://phabricator.kde.org/D7614