add cli flag --alpha-layer

now that we have long-opts, we don't have to worry about exhausting the
alphabet list for short-opts. so adding a cli flag to set/unset the
checker background makes sense.

ref: https://codeberg.org/nsxiv/nsxiv/issues/404

etc/nsxiv.1

@@ -119,6 +119,11 @@ key-handler and the input of \-i will be separated by a NULL character.
 Enables anti-aliasing, when given
 .I no
 as an argument, disables it instead.
+.TP
+.BI "\-\-alpha\-layer" [=no]
+Enables checkerboard background for alpha layer, 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: