EogThumbView: Fix visible range callback being called to often

The callback that handles unloading invisible thumbnails is prepared
to only queue the worker function once as idle source.
However, this protection was not used correctly which could cause the
callback queuing the worker multiple times. This could lead to unnecessary
redraw cycles and flickering as mentioned in !42.
1 job for bugfix/thumb-view-vis-over-queueing in 9 minutes and 52 seconds
Status Job ID Name Coverage
  Test
passed #795022
flatpak
flatpak

00:09:52