Add blurhash MSC

proposals/xxxx-blurhash-for-media.md

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+# MSC2445: Using BlurHash in Media Events
+
+[BlurHash](https://blurha.sh) is a compact representation of a placeholder
+for an image (or the frame of a video). Currently in Matrix, clients must
+display a placeholder image in the message timeline while a piece of media is
+loading. Some clients, such as Riot, simply display an empty space.
+
+While thumbnails exist to combat this to some degree, they still need to be
+downloaded from a homeserver, which is not instantaneous.
+
+Instead, a blurhash can be sent inside the `m.room.message` event, which upon
+receipt other clients can render for a pretty preview while the actual
+thumbnail downloads.
+
+## Proposal
+
+A new field is added in `m.room.message`'s `content` called `blurhash`. It
+is a BlurHash of the original piece of media. Clients could then render this
+using [one of the available BlurHash
+implementations](https://github.com/woltapp/blurhash).
+
+This would be displayed while the thumbnail of the media is loaded in parallel.
+
+This is beneficial as it's an extremely efficient way to give someone a quick
+idea of what a piece of media contains before any requests are made to a
+media repository.
+
+To be clear: This does not replace thumbnails - it will be shown before they
+are downloaded.
+
+## Visualisation
+
+Viewing an image message that is loading:
+
+![blurhashed preview](images/2445-blurhash.png)
+
+Once the image loads:
+
+![the image has loaded](images/2445-loaded-image.png)
+
+For reference, the current state of things in Riot is:
+
+![boo, sad](images/2445-current-state.png)
+
+## Alternatives
+
+We could include a base64 thumbnail of the image in the event, but blurhash
+produces much more efficient textual representations.
+
+## Backwards compatibility
+
+Older clients would ignore the new `blurhash` parameter.
+
+Newer clients would only show it if it exists.