Support transparant svg's in full window preview

[Migaroez]

Prerequisites

• I have added steps to test this contribution in the description below

Fixes #16749

Description

This PR adds a css checkered background, as is quite standard for indicating full transparency, to the full window SVG rendering that replaced the direct linking to SVG's because of the security issue described in #6182

Testing

• Create a new media item of type SVG
• Upload an SVG with transparent elements or background to said item and save it.
• go to the info app/tab and click on the link
• the transparent parts of the SVG should be rendered as a checkered pattern