Platform-Agnostic EXIF Saving via Files Tab

[yoshiyoshyosh]

SaveExif currently hooks WindowsPlatformConnector's NotifyOfScreenshot for saving screenshots with EXIF. This, of course, poses a problem for Linux users where that isn't available. Additionally, the connection with steam on Linux is hit or miss at best, and I have gotten many more misses than hits, so screenshot export via that is not a viable option.

This leaves only one way to reliably export screenshots on Linux: holding the image, going to the Files tab, and clicking the + icon. This goes through FrooxEngine.TextureExportable.Export, which goes through TextureEncoder.ConvertToPNG if png is selected (probably the preferred format for screenshots). If possible, I would really like to be able to export EXIF this way. Perhaps if there's a PhotoMetadata component on the same object, use the metadata from that?