Show a dimmed loading icon while previews are being loaded in a
background thread. The asset shelf and asset/file browsers do this
similarly already.
This is implemented in drawing code, so the loading icon will always
appear when an in-progress preview is being drawn. I experimented with
doing this in `ui_def_but_icon()`, but this won't update correctly with
popups that don't support full refreshing.
This also makes any normal icon that is drawn as preview use the normal
icon size. These icons are usually made for smaller sizes and look very
outblown when displayed at the size of a preview. Yet it's useful to
sometimes pass a normal icon. E.g. for the asset shelf we would already
draw the data-block type icon in place of the preview if there was no
preview to display, and we'd use the normal, smaller size already.
Larger can still be drawn differently.
I don't know of any current cases this would affect though.
Pull Request: https://projects.blender.org/blender/blender/pulls/133880
1f88645728