cwebp: centralize WebPCleanupTransparentArea()

this can be safely called whether the picture has alpha or not

Change-Id: I0047fd4c110f1072f183f3d340682502bd6623d7

examples/cwebp.c

@@ -876,6 +876,9 @@ int main(int argc, const char *argv[]) {
     goto Error;
   }
   picture.progress_hook = (show_progress && !quiet) ? ProgressReport : NULL;
+  if (keep_alpha == 2) {
+    WebPCleanupTransparentArea(&picture);
+  }
 
   if (verbose) {
     const double read_time = StopwatchReadAndReset(&stop_watch);