Support rendering of SVG on mobile

[Minim]

The SVG’s built-in alttext and extended description, where present, could be rendered inline:

Accessibility: Support Alt Text for Uploads and Stickers

The libexiv2-27 software library seems to be capable of manipulating the IPTC alt text and extended desc properties. ‘apt rdepends libexiv2-27’ suggests that the Gimp indeed does not have this functionality, which is bad, but at least other packages like Krita and a number of image viewers might. There are also AI tools claiming they can semi-automate adding descriptions. SVGs, whose inline display in Deltachat is not really supported due to currently-inadequate standardization, also have some …