Input system (examples): Key modifiers releases not recognized when window doesn't have focus

[markusobi]

When ALT+TABing out & in on the example applications (i.e. 27-terrain), the camera will no longer move, when pressing WASD until you press ALT again.

Reason:
When holding any modifier key (ALT/SHIFT/...) while the window loses focus (tab out/ click outside), the modifier flags will not be cleared when releasing the modifier key while the window does not have focus. When refocusing the window, the modifier will still be set.

Another way to reproduce:

• hold shift
• click out of the window
• release shift
• refocus window
• now SHIFT is still set when pressing any normal key (SHIFT + WASD doesn't move camera and shouldn't)

Tested with Example 27 & 29.