fix access of files relative to the program directory

[TomaSajt]

I had some issues with the res directory not being found correctly when my working directory was somewhere else than the project.

For example, doing dotnet run --project Fushigi/Fushigi.csproj from the root of the repository when you don't have an imgui.ini file in your current directory will crash the app.

If you have -c Release set, it will silently crash

The problem is with paths being relative to the working directory instead of the app directory (where the res directory and the .dll files are)

I saw that there was a line in the code which prepended AppDomain.CurrentDomain.BaseDirectory before "res"
and there was another line which prepended AppContext.BaseDirectory
These both solved the problem

According to this, there's no difference between them
https://learn.microsoft.com/en-us/dotnet/api/system.appcontext.basedirectory

Sadly I can't really test it out, as I couldn't properly setup my game files...
I'd be happy if someone could test if this works as expected