Load icon during rendering backend setup.

Creating a new window along with setting up an OpenGL context is universally
a lengthy operation. Decode the icon image on a separate thread.

profiler/src/main.cpp

@@ -213,10 +213,13 @@ int main( int argc, char** argv )
         } );
     } );
 
+    auto iconThread = std::thread( [] {
         iconPx = stbi_load_from_memory( (const stbi_uc*)Icon_data, Icon_size, &iconX, &iconY, nullptr, 4 );
+    } );
 
     ImGuiTracyContext imguiContext;
     Backend backend( title, DrawContents, &mainThreadTasks );
+    iconThread.join();
     backend.SetIcon( iconPx, iconX, iconY );
     bptr = &backend;