Fix truncation calculation in ASTextNode by removing layoutManager.usesFontLeading = YES in renderer context

[samhsiung]

After debugging #930 it was discovered that increasing the constrainedSize height slightly for ASTextKitRenderer's ASTextKitContext enabled it to truncate on the correct line rather than line N - 1.

After digging some more, found that ASTextKitContext's _layoutManager's usesFontLeading was set to NO. However we are using customized font leading in our project (though I'm not sure about ASTextNode in general), so this resulted in a discrepancy between the calculated shrunk size of the text node and the layout+truncation calculation.

I suspect it should be okay to remove overriding of usesFontLeading since apple docs state that the default is YES. I also think it is better to customize via the layoutManagerFactory parameter in ASTextKitContext's init method if usesFontLeading needs to be set to NO rather than overriding it for all cases.

Regardless, removing usesFontLeading = NO fixed #930 for me:

Before:

After:

[samhsiung]

Actually, we're using customized line spacing at the paragraph level and not a customized font leading so I think it's some other issue.