Fix RawCachingGlyphStore performing operations on same files from multiple threads
Fix RawCachingGlyphStore performing operations on same files from multiple threads

As seen in
https://ci.appveyor.com/project/peppy/osu/builds/37847069/tests.

To test this, modifying `TestSceneSpriteText` to [load asynchronously](https://gist.github.com/peppy/7d48f0fdb6cb138306f7769969447d0c) can
work quite well (will result in corruption of characters if not working
correctly)

smoogipooo/osu-framework,ZLima12/osu-framework,smoogipooo/osu-framework,peppy/osu-framework,ppy/osu-framework,peppy/osu-framework,ppy/osu-framework,ZLima12/osu-framework,peppy/osu-framework,ppy/osu-framework