allow disabling anti-aliasing via cli flag

simply running nsxiv with `--anti-alias` will enable anti-aliasing, and
running it with `--anti-alias=no` will disable it.

the cli flag will overwrite the config.h default.

Closes: https://codeberg.org/nsxiv/nsxiv/issues/349

etc/nsxiv.1

@@ -114,6 +114,11 @@ Set zoom level to ZOOM percent.
 .B "\-0, \-\-null"
 Use NULL-separator. With this option, output of \-o and file-list sent to the
 key-handler and the input of \-i will be separated by a NULL character.
+.TP
+.BI "\-\-anti\-alias" [=no]
+Enables anti-aliasing, when given
+.I no
+as an argument, disables it instead.
 .SH KEYBOARD COMMANDS
 .SS General
 The following keyboard commands are available in both image and thumbnail modes: