Sprite Encoding/Cache/Palette Check performance is poor due to generic.find_file

Pointing procmon at nmlc on windows reveals that before every actual read of a sprite png file, there's a full enumeration of the directory that sprite contains as nmlc attempts to normalise file case and check for potential duplicates/name collisions.

Obviously on windows this is really a nonsense, but it does impact performance in grfs that have a large number of input files in a directory. In my tests (boring hardware, large grf) 75% of the process time was apparently spent under find_file.

I think the obvious answer here is to simplify find_file to not handle case conversion at all (demand authors write better nml). Everything else is case sensitive.

Start at generic.find_file, which nmlc calls before reading sprite PNG files, and trace how it normalizes case and checks duplicate or name collisions. Use procmon with a large GRF to compare directory enumeration before and after the change; done means avoiding the unnecessary enumeration while preserving the intended case-sensitive file behavior.