Fix Windows file memory mapping to work with large files (>2GB).
Fix Windows file memory mapping to work with large files (>2GB).

Fix thanks to Steve Booth.

Also, remove a gratuitous and inefficient string copy of the mapped file.

mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3