Clear shaped brush material on command.

The initial material is held in the factory, but if a brush was previously bound, that pattern is used.
Now, using `//br set` will clear the previous material, allowing the OperationFactory's material to work.
This can be changed later with `/mat`, which will once again set the fill on the tool, overriding the factory's context.

worldedit-core/src/main/java/com/sk89q/worldedit/command/composition/ShapedBrushCommand.java

@@ -74,6 +74,7 @@ public class ShapedBrushCommand extends SimpleCommand<Object> {
             WorldEdit.getInstance().checkMaxBrushRadius(radius);
             BrushTool tool = session.getBrushTool(player.getItemInHand(HandSide.MAIN_HAND).getType());
             tool.setSize(radius);
+            tool.setFill(null);
             tool.setBrush(new OperationFactoryBrush(factory, regionFactory), permission);
         } catch (MaxBrushRadiusException | InvalidToolBindException e) {
             WorldEdit.getInstance().getPlatformManager().getCommandManager().getExceptionConverter().convert(e);