Richer input events

This expands input events to represent sub-pixel mouse positions, devices responsible for generating events, and raw
device-oriented events. The X11 back end is refactored to make full use of the new expressiveness. Other backends have
had new functionality minimally stubbed out, save for the macos backend which already supports sub-pixel mouse
positions.

src/events.rs

@@ -1,12 +1,16 @@
 use std::path::PathBuf;
-use WindowId;
+use {WindowId, DeviceId, AxisId, ButtonId};
 
 #[derive(Clone, Debug)]
 pub enum Event {
     WindowEvent {
         window_id: WindowId,
         event: WindowEvent,
-    }
+    },
+    DeviceEvent {
+        device_id: DeviceId,
+        event: DeviceEvent,
+    },
 }
 
 #[derive(Clone, Debug)]
@@ -35,31 +39,36 @@ pub enum WindowEvent {
     Focused(bool),
 
     /// An event from the keyboard has been received.
-    KeyboardInput(ElementState, ScanCode, Option<VirtualKeyCode>, ModifiersState),
+    KeyboardInput { device_id: DeviceId, input: KeyboardInput },
 
     /// The cursor has moved on the window.
     ///
-    /// The parameter are the (x,y) coords in pixels relative to the top-left corner of the window.
-    MouseMoved(i32, i32),
+    /// `position` is (x,y) coords in pixels relative to the top-left corner of the window. Because the range of this
+    /// data is limited by the display area and it may have been transformed by the OS to implement effects such as
+    /// mouse acceleration, it should not be used to implement non-cursor-like interactions such as 3D camera control.
+    MouseMoved { device_id: DeviceId, position: (f64, f64) },
 
     /// The cursor has entered the window.
-    MouseEntered,
+    MouseEntered { device_id: DeviceId },
 
     /// The cursor has left the window.
-    MouseLeft,
+    MouseLeft { device_id: DeviceId },
 
     /// A mouse wheel movement or touchpad scroll occurred.
-    MouseWheel(MouseScrollDelta, TouchPhase),
+    MouseWheel { device_id: DeviceId, delta: MouseScrollDelta, phase: TouchPhase },
 
-    /// An event from the mouse has been received.
-    MouseInput(ElementState, MouseButton),
+    /// An mouse button press has been received.
+    MouseInput { device_id: DeviceId, state: ElementState, button: MouseButton },
 
     /// Touchpad pressure event.
     ///
     /// At the moment, only supported on Apple forcetouch-capable macbooks.
     /// The parameters are: pressure level (value between 0 and 1 representing how hard the touchpad
     /// is being pressed) and stage (integer representing the click level).
-    TouchpadPressure(f32, i64),
+    TouchpadPressure { device_id: DeviceId, pressure: f32, stage: i64 },
+
+    /// Motion on some analog axis not otherwise handled. May overlap with mouse motion.
+    AxisMotion { device_id: DeviceId, axis: AxisId, value: f64 },
 
     /// The window needs to be redrawn.
     Refresh,
@@ -73,6 +82,48 @@ pub enum WindowEvent {
     Touch(Touch)
 }
 
+/// Represents raw hardware events that are not associated with any particular window.
+///
+/// Useful for interactions that diverge significantly from a conventional 2D GUI, such as 3D camera or first-person
+/// game controls. Many physical actions, such as mouse movement, can produce both device and window events. Because
+/// window events typically arise from virtual devices (corresponding to GUI cursors and keyboard focus) the device IDs
+/// may not match.
+///
+/// Note that these events are delivered regardless of input focus.
+#[derive(Clone, Debug)]
+pub enum DeviceEvent {
+    Added,
+    Removed,
+    Motion { axis: AxisId, value: f64 },
+    Button { button: ButtonId, state: ElementState },
+    Key(KeyboardInput),
+    Text { codepoint: char },
+}
+
+#[derive(Debug, Clone, Copy)]
+pub struct KeyboardInput {
+    /// Identifies the physical key pressed
+    ///
+    /// This should not change if the user adjusts the host's keyboard map. Use when the physical location of the
+    /// key is more important than the key's host GUI semantics, such as for movement controls in a first-person
+    /// game.
+    pub scancode: ScanCode,
+
+    pub state: ElementState,
+
+    /// Identifies the semantic meaning of the key
+    ///
+    /// Use when the semantics of the key are more important than the physical location of the key, such as when
+    /// implementing appropriate behavior for "page up."
+    pub virtual_keycode: Option<VirtualKeyCode>,
+
+    /// Modifier keys active at the time of this input.
+    ///
+    /// This is tracked internally to avoid tracking errors arising from modifier key state changes when events from
+    /// this device are not being delivered to the application, e.g. due to keyboard focus being elsewhere.
+    pub modifiers: ModifiersState
+}
+
 #[derive(Debug, Hash, PartialEq, Eq, Clone, Copy)]
 pub enum TouchPhase {
     Started,
@@ -98,13 +149,14 @@ pub enum TouchPhase {
 ///
 /// Touch may be cancelled if for example window lost focus.
 pub struct Touch {
+    pub device_id: DeviceId,
     pub phase: TouchPhase,
     pub location: (f64,f64),
     /// unique identifier of a finger.
     pub id: u64
 }
 
-pub type ScanCode = u8;
+pub type ScanCode = u32;
 
 #[derive(Debug, Hash, PartialEq, Eq, Clone, Copy)]
 pub enum ElementState {