PathMap: Be more generous with built-in fragment buffer

Increased the size of the fragment buffer allocated with the PathMap
instance to 24 fragments. Now that PathMap no longer has an integral
path buffer (was 128 bytes) that space is now made available for the
mapping of fragments.

doomsday/engine/portable/include/pathmap.h

@@ -34,7 +34,7 @@ typedef struct pathmapfragment_s {
 } PathMapFragment;
 
 /// Size of the fixed-length "short" fragment buffer allocated with the map.
-#define PATHMAP_FRAGMENTBUFFER_SIZE 8
+#define PATHMAP_FRAGMENTBUFFER_SIZE 24
 
 typedef struct pathmap_s {
     const char* path; ///< The mapped path.