don't multiply svg sizes by devicepixelratio

Summary:
when the native high dpi scaling is enabled, some things gets scaled two times,
as sizes taken from svgs should *not* be manually scaled by devicePixelRatio

Test Plan:
works correctly on wayland, svg.elementSize will have to be tweaked in x11 to
return a value scaled by scalefactor, to homogenize the behavior compared to
native qt high dpi scaling.
to return a value scaled by scalefactor for this qml code to work there too.
I advise everybody with high dpi laptops to test this on both wayland and x11

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: hein, broulik, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9286