Fixes #442: NPE when calling background(PIamge)

Issue documented at #442

Kf

Author: kfrajer

core/src/processing/opengl/PGraphicsOpenGL.java

@@ -5792,6 +5792,7 @@ protected void setImpl(PImage sourceImage,
 
     // Copies the pixels
     loadPixels();
+    if(sourceImage.pixels==null) sourceImage.loadPixels();
     int sourceOffset = sourceY * sourceImage.pixelWidth + sourceX;
     int targetOffset = targetY * pixelWidth + targetX;
     for (int y = sourceY; y < sourceY + sourceHeight; y++) {