Rotate View: a two-finger rotation on a Mac trackpad turns the view about the pointer

A two-finger rotation on a Mac trackpad did not turn the view. With
the new General preference Trackpad Gesture to Rotate View
(`trackpadRotateView`, on by default) and the pointer over the canvas,
the rotation turns the view about the pointer, keeping the document
point under it in place. The view does not turn until the rotation
reaches 5°; it then turns by the whole rotation and follows each later
rotation event. A rotation ends 0.25 s after its last event. A rotation
in the same frame as a pinch zooms and turns about the same point.

egui reports the gesture as `Event::Rotate`: egui-winit makes it from
winit's macOS rotation gesture, and eframe makes it from Safari's
gesture events in the web build. Rotation events that arrive while
fingers are on a touch screen do not turn the view. With the
preference off, the gesture does not turn the view.

Docs: docs/mcp.md, docs/target-app-parity.md, docs/hardware-parity.md.

crates/engine/src/cmd/prefscmds.rs

@@ -138,6 +138,7 @@ pub const PREF_SPECS: &[PrefSpec] = &[
     p!("scaleStrokes", "General", "Options", "Scale Strokes & Effects", bool),
     p!("zoomWithMouseWheel", "General", "Options", "Zoom with Mouse Wheel", bool),
     p!("scrubNumericFields", "General", "Options", "Scrub Numeric Fields by Dragging", bool),
+    p!("trackpadRotateView", "General", "Options", "Trackpad Gesture to Rotate View", bool),
     // Selection & Anchor Display
     p!("selectionTolerance", "Selection & Anchor Display", "Selection", "Tolerance", num(1.0, 8.0, "px")),
     p!("objectSelectionByPathOnly", "Selection & Anchor Display", "Selection", "Object Selection by Path Only", bool),