SDL_MouseWheelEvent does not provide the mouse location, so we used
SDL_GetMouseState() to retrieve it.
Unfortunately, SDL_GetMouseState() returns a position expressed in the
window coordinate system while the position filled in SDL events are
expressed in the renderer coordinate system. As a consequence, the
scroll was not applied at the right position on the device.
Therefore, convert the coordinate system.
See <https://stackoverflow.com/questions/49111054/how-to-get-mouse-position-on-mouse-wheel-event>.
The "screen control" handled user input, which happened to be only
used to control the screen.
The controller and screen were passed to every function. Instead, group
them in a struct input_manager.
The purpose is to add a new shortcut to enable/disable FPS counter. This
feature is not related to "screen control", and will require access to
the "frames" instance.
