force "-pass 6" if -psnr or -size is used but -pass isn't.

This is to prevent users shooting in the foot using -psnr or
-size alone and not getting the expected result.

Change-Id: I67a3289e4ec0a2a813c98807f2ec5e600f52dc63

examples/cwebp.c

@@ -972,6 +972,11 @@ int main(int argc, const char *argv[]) {
                       " encoding. Ignoring this option!\n");
     }
   }
+  // If a target size or PSNR was given, but somehow the -pass option was
+  // omitted, force a reasonable value.
+  if (config.target_size > 0 || config.target_PSNR > 0) {
+    if (config.pass == 1) config.pass = 6;
+  }
 
   if (!WebPValidateConfig(&config)) {
     fprintf(stderr, "Error! Invalid configuration.\n");