Don't allow svg images as screenshots

[razzeee]

SVG images shouldn't be allowed as screenshots, I don't think that makes sense.

E.g. https://gitlab.freedesktop.org/rncbc/qpwgraph/-/blob/46933a4f67816c75a57a1f414c28ec1f9ca76df5/src/appdata/org.rncbc.qpwgraph.metainfo.xml#L24 should fail

[ximion]

Yeah, this is actually completely unsupported. This would fail with an error in appstreamcli compose or appstream-generator, but we can have at least a rudimentary check in the regular validator as well :-)

[bbhtt]

This would fail with an error in appstreamcli compose

It doesn't https://flathub.org/apps/org.rncbc.qpwgraph

[ximion]

Wait, we just render it? I'm impressed! I guess I removed some restrictions when rewriting the compose code in C 😅

[bbhtt]

I blocked it in linter last week and haven't seen any complaints.

[ximion]

Yeah, SVG isn't allowed for screenshots - although we are a bit more lenient with the formats we accept in MetaInfo (unlike generated AppStream Catalog data where the supported output images are strictly defined).