Increase limit for open files at the stream I/O level on Windows (#6756)

elsidrequested to merge
elsid/openmw:increase_open_files_limit_on_windows into master

Mitigate #6756 (closed) caused by !1776 (merged) on Windows only. Apparently std::ifstream uses stream level I/O in the MSVC STL so it's impossible to have as many as CreateFile can open files. See this for details.

Not really a fix because Linux couldn't keep open files as much as Windows until !1776 (merged) anyway.

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