Fix rubber band selection drawing contents misaligned with scaling
Summary:
in drawPixmap(source, sourceRect) the source rect is in native pixels.
In this code the size was scaled by the DPR, but the location was not.
blendRect is the contents of a selection rectangle, offset from the
bounding darker blue rectangle. Hence we haven't really noticed.
This completely fixes the rectangle when the scale == 2.0/3.0
Some smaller artifacts remain on fractional scale factors, which is a
separate code bug with the same symptoms.
CCBUG: 386111
Reviewers: aacid, rkflx
Reviewed By: rkflx
Subscribers: sander, rkflx, anthonyfieroni, Okular
Tags: Okular
Differential Revision: https://phabricator.kde.org/D8980