X64-Question in TGpHugeFile: BufOffs maybe too short

```
In Unit GpHugeF.pas

the class (and some functions) use a Buffer-Offset. But this offset is defined
as DWORD.
Is there any possibility in X64 and very large files (I have >3GB-Files), that
the BufOffset is too small?

TGpHugeFile = class
private
hfAsynchronous : boolean;
hfBlockSize : DWORD;
(...)
hfBufFilePos : HugeInt; //cached FilePos (according the current hfBufOffs)
hfBufOffs : DWORD; //current position inside the buffer

Suggestion:
hfBufOffs : HugeInt;

```

Original issue reported on code.google.com by `mxxa2a...@gmail.com` on 25 Mar 2015 at 8:58

Start with Unit GpHugeF.pas and inspect TGpHugeFile and the functions that use hfBufOffs, especially how buffer offsets relate to hfBufFilePos and large-file positions. Determine whether DWORD can truncate valid offsets on x64 or with files larger than 3 GB; done means documenting the result and changing the type only if the code and platform behavior require it.

Written by the indexing model from the issue text.