Resolved FileNotFoundErrors encountered in pymapconv.py

[DriesDD]

• minimapthread subprocess.Popen now uses the shell to resolve FileNotFoundError
• ensure that Temp directories are created to prevent errors like FileNotFoundError: [Errno 2] No such file or directory: 'temp\thread2\temp6.BMP'
• Fixed another FileNotFoundError related to nvdxt.exe by rewriting the path code

[DriesDD]

This is my fix to use the script by fixing errors in the order I encountered them. However, I am not familiar with the project so this could use testing/review.

Also, there is one final error from line 966 with minimap.dds not being found that I could resolve by changing the code to look for minimap.bmp which is generated correctly, and while the script now runs to completion it still doesn't generate a working minimap so I don't want to commit that. I guess the .dds extension is not a mistake but the file is not created properly.

[Beherith]

The minimap dds binary blob gets embedded into the .smf output file, and is thus required for successful compilation.