[cairo] char* vs. unsigned char*

[Bill Spitzak]

Strings (including UTF-8) are "char*" in all arguments. If treating it 
as unsigned is useful, I usually do "unsigned char c = *(unsigned 
char*)pointer" and then work with c (ie do things like "if (c<0xa0)"), 
though I have no idea if this makes the best compiled code.

Images are "unsigned char*" for obvious reasons (so that a>b means a is 
brighter).

Don't use "signed char" for anything.