Fix bookmark icon display on icon border with low dpi

Sadly the MarkInterface currently only supports pixmaps, not icons.
And the Breeze bookmark 32x32 icon used for the bookmark mark has 1 pixel
wide strokes, which vanish if scaled with Qt::FastTransformation to mark
sizes of < 16x16 (like can happen on lowdpi screens with 10pt text fonts)
Other icons (like KDevelop breakpoint icon) also look better if scaled
smoothly.