Stacked opaque images should not show multiple hover overlays

[Wumpf]

Two cases where we show too many hover overlays for images right now:

• fully opaque images (configured accordingly)
• image types that don't support transparency to begin with (depth image!)

This is rather confusing since it's not like we'd show points behind a mesh on hover either!

Came up in

• 2D view of multiple DepthImages doesn't update with time scroll #7679