Merge: gamnit: abstract camera scroll and zoom service

The new service `accept_scroll_and_zoom` lets the user adjust the camera using platform specific controls. On desktop computers, the mouse wheel controls the zoom and the middle mouse button drags the world/camera. On Android, a pinch gesture controls the zoom and 2 fingers slide the scroll.

This reuse the previous `accept_two_fingers_motion` and intro an implementation for desktop computers. It will be used by, at least, Sputnit in which it will replace the current implementation which is broken since the upgrade to SDL2.

Pull-Request: #2407

diff --cc lib/gamnit/flat.nit
index 027d349,c9935d8..d59d7e3
--- 1/lib/gamnit/flat.nit
--- 2/lib/gamnit/flat.nit
+++ b/lib/gamnit/flat.nit
@@@ -41,10 -41,8 +41,9 @@@ import performance_analysi
  
  import gamnit
  import gamnit::cameras
 +import gamnit::dynamic_resolution
  import gamnit::limit_fps
- 
- import android_two_fingers_motion is conditional(android)
+ import gamnit::camera_control
  
  # Draw a `texture` at `center`
  #