Copying student sketch from one slie slide to the next

I want the students to create a sketch and on the next slide see their sketch and be able to add to it.

Something like this:
background: sketchLayer(sketch1.sketch)

For more than one sketch:
background: sketchLayer(mergeSketches(sketch1.sketch,sketch2.sketch))

I use background: sketchLayer(sketch1.sketch) often to carry over student work, but it only carries over the drawing and not any text that students type in. If I want students to add typed values on a sketch, how can I get that to carry over to the next screen?