Blurhash pulsates showing the thunbnail ~3 times before actually revealing the thumbnail on slow connection

On a bad data connection, on today's nightly, I switched to a room with a large screenshot in it. The blurhash faded away sufficiently for me to read the text in the screenshot... and then faded back in again. This repeated about 3 times over a second or so until it finally disappeared to show the thumbnail. One possibility is that the thunbnail was progressively loading on the slow connection, causing the weird striptease effect.

For progressive loading, perhaps we can start the image at 0% opacity and animate that up to 100% as soon as we start to get loading (rather than loaded) callbacks. If the image loads quickly then it will be a smooth transition from blurhash to thumbnail. If the image loads slowly however we'll get a smooth transition to the progressively loading image, which will then cause a hard vertical wipe effect as it loads over the top of the blurhash... but at least that'll be better than the current behaviour.