Automatically create user_gfx directory.

[shacknetisp]

Summary

SUMMARY: Infrastructure "Automatically create user_gfx directory"

Purpose of change

In #35091 I overlooked automatic creation of the user_gfx directory. Automatically creating this directory will make things more obvious about where to put tilesets. This behavior already exists for the user_sounds directory.

Describe the solution

This is a simple addition to create the gfx/ user directory like sounds/ is already created

Testing

I started the game with no gfx/ directory in my user directory, it was created on startup.