Wheel deltas were truncated to int at the platform input boundary,
which dropped the sub-pixel tail of trackpad momentum scrolls. Each
NSEvent's scrollingDeltaY arrived as a CGFloat, got cast to int, and
flowed through IPC, EventHandler, and PaintableBox::scroll_by as int,
losing fractional information that never came back.
Widen Web::MouseEvent::wheel_delta_{x,y} to double and propagate
through Page, EventHandler, Paintable, PaintableBox, and the AppKit,
Qt, and GTK input paths.
9f35827aa0