Revert "Remove unused data field in RenFont"

This reverts commit 461266e97d.

Related to github issue #122.

The void *data field is not used but the glyphset index can
someting be equal to -1 and the unused field prevent faulty
writes before the allocated struct.

src/renderer.c

@@ -19,6 +19,7 @@ struct GlyphSet {
 typedef struct GlyphSet GlyphSet;
 
 struct RenFont {
+  void *data;
   GlyphSet *sets[MAX_GLYPHSET];
   float size;
   int height;