Failure to detect the tileset key color when the zeroth tile contains a custom glyph #157

Describe the bug
When loading a tileset libtcod will fail to detect the key color if the zeroth tile has a unique glyph. This will load the font as a multi-color font instead of removing the color key.

To Reproduce
Load any tileset with a non-void zeroth tile such as Aesomatica_16x16.png
https://dwarffortresswiki.org/index.php/DF2014:Tileset_repository#16.C3.9716

Expected behavior
Libtcod should still be able to detect the key color from the space glyph.

More info
Issue discovered by marssaxman on Reddit