Fix HiDPI vs. set_cursor_icon for web (#1652)

PhysicalSize is recorded as canvas.size, whereas LogicalSize is stored
as canvas.style.size.

The previous cursor behavior on stdweb clobbered all style - thus losing
the LogicalSize.

src/platform_impl/web/window.rs

@@ -165,8 +165,7 @@ impl Window {
             CursorIcon::RowResize => "row-resize",
         };
         *self.previous_pointer.borrow_mut() = text;
-        self.canvas
-            .set_attribute("style", &format!("cursor: {}", text));
+        backend::set_canvas_style_property(self.canvas.raw(), "cursor", text);
     }
 
     #[inline]