Add time since pointerdown to input event handling (#4443)

Add event time to the editor pointer states

The brush trace's hot core is position plus per-sample time, and the
frame clock is too coarse for high-rate tablets: several samples share
one timestamp and speed dynamics see dt = 0. The desktop stamps
editor-routed pointer input with a monotonic arrival time since winit
does not surface hardware timestamps; the web path can thread the
PointerEvent timeStamp through later.

desktop/src/input.rs

@@ -6,6 +6,7 @@ use crate::ui::{MULTICLICK_ALLOWED_TRAVEL, MULTICLICK_TIMEOUT, PINCH_ZOOM_SPEED,
 use crate::wrapper::messages::{DesktopWrapperMessage, InputMessage, ModifierKeys, MouseKeys, PointerState, ScrollDelta};
 
 pub(crate) struct InputState {
+	start: Instant,
 	viewport_info: Option<ViewportInfo>,
 	pointer_lock_position: Option<PhysicalPosition<f64>>,
 	modifier_keys: ModifierKeys,
@@ -29,6 +30,7 @@ impl InputAction {
 impl InputState {
 	pub(crate) fn new() -> Self {
 		Self {
+			start: Instant::now(),
 			viewport_info: None,
 			pointer_lock_position: None,
 			modifier_keys: ModifierKeys::empty(),
@@ -193,6 +195,7 @@ impl InputState {
 		PointerState {
 			editor_position: (self.pointer_position.x / self.scale(), self.pointer_position.y / self.scale()).into(),
 			mouse_keys: self.pointer_keys,
+			time: Some(self.start.elapsed().as_secs_f64() * 1000.),
 			..Default::default()
 		}
 	}