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