[Performance] Swappy becomes laggy the more paints are made

[jtheoof]

The way the render works is to re-render all the paints every time. This makes the tool become more and more laggy as more things are being drawn. Some paints are heavier than others, for example, the new blur tool is quite intensive. Once a blur has been rendered, no need to re-render it every time, only for some undo/redo, or clear.

Need to find a better to not re-render every time. For example use the last drawn cairo surface as base for new paints and flag the rendered paints.