feat: user can disable the first wallpaper transition

[micycle8778]

Optionally removes the initial transition from black screen via initial_transition configuration option.

no initial transition config

```toml # config.toml [eDP-1] path = "/home/micycle/Downloads/wallpapers/" duration = "5s" transition_time = 2000 initial_transition = false ```

no_initial_transition.webm

initial transition config

```toml # config.toml [eDP-1] path = "/home/micycle/Downloads/wallpapers/" duration = "5s" transition_time = 2000 initial_transition = true # the default ```

initial_transition.webm

[danyspin97]

Hello @micycle8778 and thank you for your PR! I have to say that it is very polished and readable, it is going to be accepted as it is :)