The "Not Bound" scancode can activate key binds

Since switching from GLFW to SDL in 26.3 snapshots, ‘not bound’ controls (which are technically just bound to the ‘unknown’ key) are now activated whenever an ‘unknown’ key is pressed. Previously, GLFW did not send key events for unknown keys.

This is more noticeable on keyboards where SDL maps many keys to ‘unknown’, see upstream issues like https://github.com/libsdl-org/SDL/issues/16047

To reproduce:
1. ensure some controls are ‘not bound’, e.g. cinematic camera
2. press an ‘unknown’ key, e.g. Fn
3. notice the control was activated