LibWeb: Fill OOPWV with the palette base color until we have pixels

We were painting unfinished pixels while waiting for the WebContent
process to render the page. This caused OOPWV to flicker black
sometimes, which looked pretty bad.

This way we still flicker, but at least we flicker with the correct
palette color. :^)

Libraries/LibWeb/OutOfProcessWebView.h

@@ -89,6 +89,8 @@ private:
     RefPtr<WebContentClient> m_client;
     RefPtr<Gfx::Bitmap> m_front_bitmap;
     RefPtr<Gfx::Bitmap> m_back_bitmap;
+
+    bool m_has_usable_bitmap { false };
 };
 
 }