Mind the devicePixelRatio when drawing on-screen in presentation mode

Previously, when using a screen scaling factor larger than 1, the
drawings by mouse or stylus in presentation mode were slightly blocky.
The underlying cause was
a) a few integer types used for non-integer coordinates
b) the intermediate use of a QPixmap without the correct
   devicePixelRatio

Additionally, this patch uses a QPainterPath to draw the path,
rather than a sequence of individual lines.  This improves
the line quality a tiny bit, because the lines are properly
joined.

BUG: 384143

Differential Revision: https://phabricator.kde.org/D21836