There is a strange issue that appears* when Theater calls
ISpriteFrame.Frames on the R8Reader. The R8Reader uses
IEnumerable.Cast<> which behaves slower and slower, which
makes map loading become 10+ times slower.
The changes here simply avoid the casting.
[*] This happens at least on Linux x86_64 with Mono 3.2.8.
See https://bugzilla.xamarin.com/show_bug.cgi?id=19668
2ec6df9680