Add effect for a wallpaper around the terminal window

[sassman]

I have seen often times screenshots or screen recordings shared where portions of the MacOS wallpaper are shared (like below).

t-rec should support (opt in for now) to have a nice wallpaper present as well.

• the default wallpaper should be provided by t-rec and must be defined yet.
  • it should be available offline (no internet download required)
• users can specify their own wallpaper by a cli argument (-w | --wallpaper <path-to-file>).
• png, jpeg and tga should be supported
• the resolution must be higher (+10% wider and higher) than the recorded terminal is
• the resulution shold be validated before the recording starts and in cases where it does not fit the user should be informed and the t-rec stops there
• the excat dimensions for the visible wallpaper is tbd

Edit: feel free to propose some wallpapers below, they should adhere to the creative commons license at least