Fix selection rectangle for SVG images in View comparison mode

Summary:
For SVG images, the selection rect encompassed the entire bounding
rect. This patch implements SvgViewAdapter::visibleDocumentRect
so it returns the correct rect instead of falling back to the generic
implementation in AbstractDocumentViewAdapter.

Before

After

Test Plan: Open at least one SVG in View comparison mode, and select the SVG.

Reviewers: Gwenview, ngraham, rkflx

Reviewed By: Gwenview, ngraham, rkflx

Subscribers: rkflx, ngraham

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D11796