On Windows, fix left mouse button release event not being sent after Window::drag_window (#2564)

src/platform_impl/windows/window_state.rs

@@ -52,6 +52,8 @@ pub(crate) struct WindowState {
     pub is_active: bool,
     pub is_focused: bool,
 
+    pub dragging: bool,
+
     pub skip_taskbar: bool,
 }
 
@@ -162,6 +164,8 @@ impl WindowState {
             is_active: false,
             is_focused: false,
 
+            dragging: false,
+
             skip_taskbar: false,
         }
     }