FastR Grid: fix savePlot function.

com.oracle.truffle.r.library/src/com/oracle/truffle/r/library/fastrGrid/device/awt/Graphics2DDevice.java

@@ -182,6 +182,7 @@ public class Graphics2DDevice implements GridDevice {
     void setGraphics2D(Graphics2D newGraphics) {
         assert newGraphics != null;
         graphics = newGraphics;
+        cachedContext = null;
     }
 
     public Graphics2D getGraphics2D() {