[Gtk] fix image sizing

[lytico]

https://learn.microsoft.com/en-us/dotnet/maui/user-interface/controls/image?view=net-maui-8.0#control-image-scaling

issue

images are not sized correctly according to aspect

https://github.com/dotnet/maui/blob/e59620b70f01fe45c0aff5d3292f4f5b6c081e2b/src/Core/src/Platform/Gtk/ImageView.cs#L48-L117

needs a refactoring

and

https://github.com/dotnet/maui/blob/e59620b70f01fe45c0aff5d3292f4f5b6c081e2b/src/Core/src/Platform/Gtk/ImageView.cs#L136-L138

needs another implementation; call to DrawPixbuf has to be changed to be clippable if Aspect = Aspect.AspectFill