[Media Frame] Set sourceSize for higher quality and lower memory consumption
Summary: Without this, diagonal lines are aliased and many images look terrible.
Slideshow transitions still work.
Performance still seems good in a crappy VM with 2 GB RAM and software rendering only.
Reviewers: Plasma, broulik, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: anthonyfieroni, broulik, davidedmundson, plasma-devel
Differential Revision: https://phabricator.kde.org/D19550