History surfaces don't get Disposed

[jpobst]

I'm not sure if this change is correct:
8eefbe5#diff-36d2425cac4ec535e53cf124ae8a76ec1002823dece3982a68ed299224f6484a

(The link doesn't seem to go to the correct spot, but the issue is removing Dispose from BaseHistoryItem and SimpleHistoryItem.)

By removing the Dispose, old_surface doesn't get released (until the eventual finalizer runs).

This can cause unused native memory to linger longer than necessary.

[cameronwhite]

Yeah, that should likely be restored - I vaguely remember there being some issue with disposing them at the time (or maybe they weren't disposable at all?) but that's not the case anymore.