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.

CHANGELOG.md

@@ -48,6 +48,7 @@
 * Hostname in OSC 7 URI not being validated.
 * OSC 4 with multiple `c;spec` pairs.
 * Alt+Return to emit "ESC \r".
+* Trackpad sloooow scrolling to eventually scroll a line.
 
 
 ### Security