Issue description
Mouse wheel should only change camera zoom when the mouse is inside the game window
Reproduction steps
Have the game running in windowed mode, move the mouse cursor outside the game window but keep the game in focus, scroll the mouse wheel (eg when scrolling a gui element in another window), this will change the camera zoom but it shouldn't. It is a very common convention that mouse wheel scrolling only does something to a gui element that the mouse is over, actually regardless of which window has focus.
Game version and build number
v0.7.8 (b495)
Operating system
Windows
