Show high-resolution and vector logos properly in HighDPI mode

Summary:
BUG: 388633

Take the device pixel ratio into consideration when rendering the logo, so it looks nice in HighDPI mode.

Test Plan:
Tested in NDE Neon with the standard Plasma logo, which is an SVG.

Before:

After:

Reviewers: Plasma

Subscribers: cfeck, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10357