Fixed some small bugs and added calc example

Fixed window movement for windows without header and some small
wrong calculation inside the panel layout code. In addition I
added another example implementing a simple calculator.

demo/x11/main.c

@@ -124,7 +124,7 @@ main(void)
 
         /* GUI */
         {struct nk_panel layout;
-        if (nk_begin(ctx, &layout, "Demo", nk_rect(50, 50, 200, 300),
+        if (nk_begin(ctx, &layout, "Demo", nk_rect(50, 50, 200, 200),
             NK_WINDOW_BORDER|NK_WINDOW_MOVABLE|NK_WINDOW_SCALABLE|
             NK_WINDOW_CLOSABLE|NK_WINDOW_MINIMIZABLE|NK_WINDOW_TITLE))
         {

example/Makefile

@@ -32,3 +32,8 @@ extended:
 	rm -f bin/extended $(OBJS)
 	$(CC) $(CFLAGS) -o bin/extended extended.c $(LIBS)
 
+calculator:
+	@mkdir -p bin
+	rm -f bin/calculator $(OBJS)
+	$(CC) $(CFLAGS) -o bin/calculator calculator.c $(LIBS)
+