Blender Git Loki

Fix T54310: Assert when enable Brush custom icon.

We had a mix of two issues here actually:
* First, Brush are currently using their own sauce for custom previews,
this is not great, but moving them to use common ImagePreview system of
IDs is a low-priority TODO. For now, they should totally ignore their
own ImagePreview.
* Second, BKE_icon_changed() would systematically create a PreviewImage
for ID types supporting it, which does not really makes sense, this
function is merely here to 'tag' previews as outdated. Actual creation
of previews is deferred to later, when we actually need them.