FrameSvg: also clear mask cache on clearCache()

Summary:
It can be unexpected that those are not reset when clearCache() was called.

With the current logic when a theme changes the mask cache not reset also
results in wrong lookups, as the cacheid used as key does not include the
theme name.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D21172

src/plasma/framesvg.cpp

@@ -339,9 +339,11 @@ void FrameSvg::clearCache()
 {
     if (d->frame) {
         d->frame->cachedBackground = QPixmap();
+        d->frame->cachedMasks.clear();
     }
     if (d->maskFrame) {
         d->maskFrame->cachedBackground = QPixmap();
+        d->maskFrame->cachedMasks.clear();
     }
 }