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
diff --git a/examples/vp8_set_maps.txt b/examples/vp8_set_maps.txt
index 94554ba..ad9ef61 100644
--- a/examples/vp8_set_maps.txt
+++ b/examples/vp8_set_maps.txt
@@ -78,8 +78,8 @@
 } 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;