On Windows, fix RedrawRequested delivery

When calling `Window::request_redraw` from the `RedrawRequested`
handler the `RedrawWindow` won't result in `WM_PAINT` being delivered
due since user callback is run before `DefWindowProcW` is called.

Track whether the user called `Window::request_redraw` and ask for
`RedrawWindow` after running the said function during `WM_PAINT`
handling.

Fixes #3150.

src/platform_impl/windows/window_state.rs

@@ -52,6 +52,9 @@ pub(crate) struct WindowState {
     pub is_active: bool,
     pub is_focused: bool,
 
+    // Flag whether redraw was requested.
+    pub redraw_requested: bool,
+
     pub dragging: bool,
 
     pub skip_taskbar: bool,
@@ -166,6 +169,7 @@ impl WindowState {
 
             is_active: false,
             is_focused: false,
+            redraw_requested: false,
 
             dragging: false,