Add option to change the background color

Introduce a `--background-color` option that accepts hexadecimal color
codes (3-digit or 6-digit formats):

    scrcpy --background-color=#234567
    scrcpy --background-color=234567  # leading '#' is optional
    scrcpy --background-color=#567    # equivalent to #556677

Fixes #5855 <https://github.com/Genymobile/scrcpy/issues/5855>
Refs #6772 comment <https://github.com/Genymobile/scrcpy/pull/6772#issuecomment-4374943892>
PR #6807 <https://github.com/Genymobile/scrcpy/pull/6807>