Clear tailing empty lines of ft2_char.c
Add some comment.
Change ft2_char into ft2_char_cairo

ft2_char uses cairo for PNG output. Since I'd like to add another
demonstration using libPNG for PNG output, I renamed original ft2_char
into ft2_char_cairo.
Finished ft2_char_libpng.c.

ft2_char_libpng.c is like ft2_char_cairo.c, except the former using
libPNG directly for PNG output.