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
{F5776897}

## After
{F5776899}

Test Plan: Open at least one SVG in {nav 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