Persist new selected colours, even after a lost connection.

[MatthewLM]

Currently on iOS, when leaving the site and coming back later, the websocket connection is lost. The new pixel selections from pressing "I have made payment" are then removed.

When reconnecting, the selected pixels should be remembered and persist unless the underlying colour has changed.

Each pixel selected by the user will need to be remembered in a separate array. For these pixels, the old colours need to be remembered or otherwise determined. If the pixel colour of the newly downloaded canvas is the same, then the pixel should be set to the user's selected pixel. If the downloaded canvas pixel is different, then the pixel should be set to the new colour from the canvas.