Port of FS251 - Scene / include files of >2GB size may cause problems

[wfpokorny]

http://bugs.povray.org/task/251

(Scan of source still shows some fseek(), ftell() use)

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Details:

Code inspection shows that we're still using fseek() and ftell() in various places (including text file input), which can't handle file positions of 2GB and beyond (except on 64-bit linux machines); those calls need to be examined and (where appropriate) replaced with the fseek64() macro we're already defining (but currently not using), and a to-be-defined ftell64() macro.

One potential (untested) error scenario would be a scene file calling a macro that is defined at the end of a > 2GB long include file.

[c-lipka]

For the records: I'll file this as "exceptionally rare scenario, with involvement of the parser", so nothing we want to tackle in a hurry while we prepare for v3.8.0 release.