vp8_set_maps: remove hard-coded width/height

The example for disabling the active map used a hard-coded 320x240
resolution, rather than using what was passed on the command line.

Fixes #218

Change-Id: I3aed713e8aa7fcbf18dfbffd57f142b5cd9ee492

examples/vp8_set_maps.txt

@@ -78,8 +78,8 @@ if(frame_cnt + 1 == 22) {
 } else if(frame_cnt + 1 == 44) {
     vpx_active_map_t  active;
 
-    active.rows = 240/16;
-    active.cols = 320/16;
+    active.rows = cfg.g_h/16;
+    active.cols = cfg.g_w/16;
 
     /* pass in null map to disable active_map*/
     active.active_map = NULL;