Implement SkiaDisplay::flip()

src/she/skia/skia_display.h

@@ -55,6 +55,7 @@ public:
   // false if the flip couldn't be done because the display was
   // resized.
   bool flip() override {
+    m_window.invalidate();
     return true;
   }
 

src/she/win/window.h

@@ -356,6 +356,10 @@ static KeyScancode vkToScancode(int vk) {
       m_captureMouse = false;
     }
 
+    void invalidate() {
+      InvalidateRect(m_hwnd, NULL, FALSE);
+    }
+
     HWND handle() {
       return m_hwnd;
     }