Force thumbnails to be the size of the container, even if that means
upscaling them. As per comment, this will mean that the thumbnails
will sometimes be larger and a bit blurry rather than small. It looks
like this was probably a source of scroll jumps before.
As per comment, a better fix would be to be be able to know what
size thumbnails the HS will give us so we can size the containers
appropriately.
Type: defect
Regressed by: https://github.com/matrix-org/matrix-react-sdk/pull/6514
Fixes https://github.com/vector-im/element-web/issues/18307
50aa3d6c46