Better Icon of Sin addon

I don't like the Icon of Sin's texture. So, I tweaked it a little bit to my liking.

Changelog:

• Texture is overall sharper and darker
• A slight green fade is added below the 'con
• Brain lava is sharper too

Contains a version for more capable source ports, and an alternate file with Doom palette limits compatible with most other sourceports. Should work with vanilla too.