Hy Murveit authored Apr 21, 2020 and Jasem Mutlaq committed Apr 21, 2020

Summary:
Instead of creating a scaled image, let Qt do the work.
This saves half the memory when zooming 200% or 300%.
I also limited the zoom to 300% running out of memory has been
an issue for users. This change was relatively straight-forward
for the image rendering, but there was complexity with the
overlays drawn, espcially those rendering text.

Test Plan:
Load an image, zoom in. You should be limited to 300%. You should notice less than
1Gb of memory use from a ~20megapixel camera, whereas this would be over 2.5Gb
previously. Try the various overlays. They should work the same as before (some line
widths may be slightly different, but I tried to match the previous rendering.
Similarly, test focus, align, guide views, zooming in and so forth.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D28935