Get metadata that are stored as key-value pairs.
A prominent example are PNG's `tEXt` and `zTXt` chunks.
::: note In contrast to gdk-pixbuf's *option* feature, the keys do not carry prefixes like `tEXt::` or `zTXt::`.
| key |
A null-terminated string. |
|
The UTF-8 encoded value associated with `key`. |