[Media Frame] Set sourceSize for higher quality and lower memory consumption

Summary: Without this, diagonal lines are aliased and many images look terrible.

Test Plan:
Before: {F6660129, size=full}

After: {F6660131, size=full}

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

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19550