Resolve context depending picking on picking readback, not during scene build up

[Wumpf]

We should always render the same picking layer out no matter what is selected at the moment, but right now we may or may not write out InstanceKey::SPLAT for instance keys depending on whether an object or an instance is selectable. This ofc means that the picking layer needs to transfer this semantic so it can be picked up again when resolving a retrieved picking layer rectangle - 0 would mean instance 0, SPLAT means all instances. Instead, we should put this logic in the code that reads the picking layer back.

This would simplify & speed up scene setup a bit. Also, as the default picking layer instance id is zero, it would be easier/more natural to setup rendering of objects without instance ids - right now they need to use the SPLAT instance constant instead.

Related to #1893

[Wumpf]

Fixed by

• Double-click to select entity #2504