"Off" feedback not sent for virtual control elements under certain conditions

[helgoboss]

When using conditional activation in the controller compartment to switch on/off a mapping with a virtual target, I came across a bug.

• A real source RS which is used as source in a virtual controller mapping VM1 of a ReaLearn instance R but doesn't have a corresponding main mapping must not be affected by that instance R - in order to let other instances or scripts take control of it. That works!
• However, one would expect that when deactivating another virtual controller mapping VM2 with the same real source RS (this one has a corresponding main mapping), "off" feedback would be sent to RS. This doesn't happen!
• The existence of VM1 prevents ReaLearn from switching the mapping "Off". When removing VM1 (or disabling feedback), "off" feedback is sent as expected.

It's low-priority for now because in practice, we mostly have corresponding main mappings in such cases.

Steps to reproduce:

• Use "APC Key 25 controller preset for Playtime 2" together with "Grid main preset" (with scene buttons unmapped)
• Press Shift and see how the scene launch buttons don't turn the lights off when Shift released.