Watermark filter dynamic path

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Hi, I was trying to apply a different watermark path depending of the user who uploaded a photo, i tried with a runtimeConfig but i don't know how to do correctly.

I do this:

```
{% set runtimeConfig = {"watermark": {"size": 0.5, "image": premium.watermark, "position": 'bottomright' }} %}

```

In the config.yml i have this:

```
grid_thumb:
quality: 50
filters:
thumbnail: { size: [244, 244], mode: outbound }
```

This is the error in the profiler:

Signed url does not pass the sign check for path "PHOTO-ROUTE" and filter "grid_thumb" and runtime config {"watermark":{"size":"0.5","image":"WATERMARK-ROUTE","position":"bottomright"}}

Start with the template's imagine_filter('grid_thumb', runtimeConfig) usage and the grid_thumb definition in config.yml, then inspect the profiler error about signed URLs and runtime configuration. Done means a user-dependent watermark path can be passed to this filter without failing the sign check.

Written by the indexing model from the issue text.