input: slow trackpad scrolling now eventually scroll a line

Before, we converted each axis event's scroll amount to an integer and
scrolled that many lines.

However, axis events are speed sensitive - very slow scrolling will
result in events with a scroll amount that is < 1.0.

For us, this meant we never scrolled a single line. You could slow
scroll all day if you wanted, and still we would never scroll a single
line.

Fix this by aggregating the scroll amount from axis events until the
scroll amount is > 1.0, and then scroll.

wayland.h

@@ -292,6 +292,7 @@ struct wayland {
         struct timeval last_time;
 
         /* We used a discrete axis event in the current pointer frame */
+        double axis_aggregated;
         bool have_discrete;
     } mouse;