Fix SVGs overlapping selection rect in View compare mode

Summary:
Zooming an SVG in compare mode allowed the SVG to be painted outside its
parent widget `SvgImageView`, effectively overlapping the selection rect
which is drawn in `DocumentView::paint`. This patch clips all painting
done by children of `AbstractImageView` to its own shape, therefore
fixing the issue.

Before:
{F5790186, size=full}

After:
{F5790283, size=full}

Test Plan:
Open an SVG image in comparison mode.
Zoom in enough so it fills the entire document view (and therefore
can be panned).
Image should not draw over the selection rectangle.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: muhlenpfordt

Differential Revision: https://phabricator.kde.org/D11942