Determines the numeric value of a character as a hexadecimal
digit. Differs from g_unichar_xdigit_value() because it takes
a char, so there’s no worry about sign extension if characters
are signed.
Parameters
c
Type:gchar
An ASCII character.
Return value
Type:gint
If c is a hex digit (according to g_ascii_isxdigit()),
its numeric value. Otherwise, -1.