Fix set_background_opacity with increment/decrement

[kdarkhan]

There is a logic error in set_background_opacity which breaks increment/decrement handling.
This commit fixes it.

[max-baz]

Can you describe this logic error? It's not obvious to me even now 🙂

[kdarkhan]

Sure. Putting this into my config

map kitty_mod+9    set_background_opacity +0.1
map kitty_mod+8    set_background_opacity -0.1

I would expect the first shortcut to increment, and second to decrement the current opacity.
However because
opacity = background_opacity_of(os_window_id)
overrides the passed in opacity parameter, and later
opacity += float(opacity) just doubles the effective opacity instead of incrementing/decrementing it.