Previously, we had to fetch all asset previews at once. However the preview
caching system was limited at 8192 previews, so with large asset libraries, many
previews would get dropped. Instead, allow selectively requesting previews and
only request previews of items displayed on screen. This should fix the issue,
and reduce the time it takes to display brush assets in general, especially with
bigger libraries. This should also reduce memory usage quite a lot when
displaying the asset shelf, since not all asset previews are loaded anymore.
Pull Request: https://projects.blender.org/blender/blender/pulls/130105
671f08df96