Blender Git Loki

Prevent preview loading from triggering continuous redraws

The preview rendering notifiers would be continuously sent, causing
continuous redraws. In the File Browser that also happens (and I don't
think it's intentionaly) although it causes refreshes there that don't
do much and there won't be redraws unless necessary. Correcting this may
cause issues, so better to do it separately.

Issue is that the previews were fetched using the BLI_task API, and a
timer was started to send notifiers continuously. But there was nothing
to check if the tasks were done, so that the timer could be stopped.