bugfix: line width wasn't working in the PNG driver

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@59547 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/pngdriver/Line_width.c

@@ -1,9 +1,7 @@
 #include <math.h>
 #include "pngdriver.h"
 
-int linewidth;
-
 void PNG_Line_width(double width)
 {
-    linewidth = (width < 0 ? 0 : (int) floor(width + 0.5));
+    png.linewidth = (width < 0 ? 0 : (int) floor(width + 0.5));
 }