Profile picture texture atlas

[jb55]

After doing some tests with ./preview ProfilePic it's pretty clear that loading textures individually is simply too slow to render them all at once, which we may want to do for notedeck_viz

We can create a giant texture with all of the profile pictures and use that as a texture atlas. For everything else we can load it dynamically as we need it.

I found some example code that uses the same underlying texture handles in different TextureIds:

• lukas-kirschner/Exodus@f2b6286

On closing of the application we could write this atlas to disk, and then load it again on next startup?

[jb55]

This could be a feature of ImageCache

[jb55]

The slowness was a bug with choosing the wrong preview cache dir