Created Unassigned: Crash when using WriteableBitmapEx.Clear in parallel. [20005]

Using WriteableBitmapEx 1.0.8.0 from NuGet, I have some code where I'm creating a bunch of images in parrel. However, inside BitmapContext there's a static Dictionary used in the ctor - which leads to crashes when calling methods such as WriteableBitmapExtensions.Clear on different images concurrently.

Here's some code to reproduce the problem

```
Parallel.For(0, 1000, i =>
{
var bitmap = new WriteableBitmap(100, 100, 96, 96, PixelFormats.Gray16, null);
var converted = BitmapFactory.ConvertToPbgra32Format(bitmap);
converted.Clear(Colors.Aqua);
});
```