Update chess env to use PNG pieces

[zdu863]

Replace font-based pieces with a centered PNG sprite sheet for the board and captured pieces. Preserve letter fallback sizing and colors. Add light-gray backgrounds behind captured icons for contrast. No gameplay changes.

Tested native Raylib and browser rendering, including missing-PNG fallback.

[bullardji]

Are these assets copyrighted? If not, then I would support the adding of these assets, as the author of this env. Otherwise I would keep things as is. Thank you for your contribution!

[zdu863]

@bullardji Thanks! The pieces are freely licensed under CC BY-SA 3.0, which permits reuse, including commercial use. They’re from Wikimedia Commons. I’ve included the title, author credits, and source/license links in the code.