variable parsing in watermark path

Also, fixed some segfaults when watermark is not found or not an PNG
image.

Useful, when your want to change watermark dynamically and without duplicating locations.

```
location ~ /(lala|blah)/ {
    image_filter watermark;
    image_filter_watermark watermark_$1.png; # choose between watermark_lala and watermark_blah
    image_filter_watermark_position center-center;
}
```