Rebuild a suboptimal swapchain after presenting, not before

`Surface::configure` panics while a texture the surface handed out is still
alive, which wgpu says at both `configure` and `get_current_texture`. The
`Suboptimal` arm configured with the texture it was about to draw with in
hand, so the first suboptimal frame -- a resize or a display change on some
drivers -- takes the app down instead of rebuilding the swapchain.

The texture is good for this frame, so it is drawn with and presented, and
the rebuild happens once `present` has consumed it.

src/default/render.rs

@@ -22,12 +22,12 @@ impl UiRenderer {
     }
 
     pub fn draw(&mut self) {
-        let output = match self.surface.get_current_texture() {
-            CurrentSurfaceTexture::Success(texture) => texture,
-            CurrentSurfaceTexture::Suboptimal(texture) => {
-                self.surface.configure(&self.device, &self.config);
-                texture
-            }
+        let (output, suboptimal) = match self.surface.get_current_texture() {
+            CurrentSurfaceTexture::Success(texture) => (texture, false),
+            // Used for this frame, and the swapchain rebuilt after it has
+            // been presented: configuring the surface while a texture it
+            // handed out is still alive panics.
+            CurrentSurfaceTexture::Suboptimal(texture) => (texture, true),
             CurrentSurfaceTexture::Outdated | CurrentSurfaceTexture::Lost => {
                 self.surface.configure(&self.device, &self.config);
                 return;
@@ -60,6 +60,9 @@ impl UiRenderer {
         self.queue.submit(std::iter::once(encoder.finish()));
         self.window.pre_present_notify();
         self.queue.present(output);
+        if suboptimal {
+            self.surface.configure(&self.device, &self.config);
+        }
     }
 
     pub fn resize(&mut self, size: &PhysicalSize<u32>) {