pinta: FIX unhandled glib-sharp load exception
pinta: FIX unhandled glib-sharp load exception

running pinta after https://github.com/NixOS/nixpkgs/pull/31497
results in:

```
Unhandled Exception:
System.IO.FileNotFoundException: Could not load file or assembly 'glib-sharp, Version=2.12.0.0, Culture=neutral, PublicKeyToken=35e10195dab3c99f' or one of its dependencies.
File name: 'glib-sharp, Version=2.12.0.0, Culture=neutral, PublicKeyToken=35e10195dab3c99f'
[ERROR] FATAL UNHANDLED EXCEPTION: System.IO.FileNotFoundException: Could not load file or assembly 'glib-sharp, Version=2.12.0.0, Culture=neutral, PublicKeyToken=35e10195dab3c99f' or one of its dependencies.
File name: 'glib-sharp, Version=2.12.0.0, Culture=neutral,
PublicKeyToken=35e10195dab3c99f'
```

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