Watermark-Filter does not support transparency?

Nobody has claimed this yet.

When using the watermark-filter, the watermark appears with a black background on the filtered image.
These are the filter settings im using:

```
liip_imagine:
driver: gd
resolvers:
default:
web_path:
web_root: ~

loaders:
default:
filesystem:
data_root: ~

filter_sets:
watermark_image:
filters:
watermark:
image: ../data/watermark_safe.png
# size of the water mark relative to the input image
size: 1
# set the position of the watermark
position: center
```

Am I doing something wrong?

The issue names only the watermark filter configuration and watermark_safe.png; no repository file or test is identified. Start by reproducing the black background with the shown settings, then locate the watermark filter implementation and its transparency handling. Done means determining whether the configuration is valid and verifying correct transparent-watermark behavior with a regression test.

Written by the indexing model from the issue text.