Fix strange d2k loading slowness

Note: I tracked this down to being a Mono bug that was fixed somewhere between  3.2.8 and 3.4.0. I don't know if you want this merged or not.

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.