Image preview related options should not be set if image preview is not requested

[097115]

May be something like this could help:

-        [ -z "$preview_window_size" ] && preview_window_size=up:60%:wrap
-        [ -z "$ueberzug_x" ] && ueberzug_x=10
-        [ -z "$ueberzug_y" ] && ueberzug_y=3
-        [ -z "$ueberzug_max_width" ] && ueberzug_max_width=$(($(tput lines) / 2))
-        [ -z "$ueberzug_max_height" ] && ueberzug_max_height=$(($(tput lines) / 2))
+        ! [ "$image_preview" -eq 0 ] && [ -z "$preview_window_size" ] && preview_window_size=up:60%:wrap
+        ! [ "$image_preview" -eq 0 ] && [ -z "$ueberzug_x" ] && ueberzug_x=10
+        ! [ "$image_preview" -eq 0 ] && [ -z "$ueberzug_y" ] && ueberzug_y=3
+        ! [ "$image_preview" -eq 0 ] && [ -z "$ueberzug_max_width" ] && ueberzug_max_width=$(($(tput lines) / 2))
+        ! [ "$image_preview" -eq 0 ] && [ -z "$ueberzug_max_height" ] && ueberzug_max_height=$(($(tput lines) / 2))

Or, more probably, there's a better way :)

Of course, setting those even if not specifically requested may not seem that harmful at first but a) with no terminal available, using tput results in further calculations may lead to an error (you can't divide nothing by 2) and b) why really do it if there's no need? :)