Fixed rendering issue with all layers hidden.
Fixed rendering issue with all layers hidden.

Since the drawing surface was never cleared, the old canvas contents
would end up being drawn if all layers were hidden.

Fenex/Pinta,Fenex/Pinta,PintaProject/Pinta,jakeclawson/Pinta,Mailaender/Pinta,jakeclawson/Pinta,Mailaender/Pinta,PintaProject/Pinta,PintaProject/Pinta