Add virtual display feature

Add a feature to create a new (separate) virtual display instead of
mirroring the device screen:

    scrcpy --new-display=1920x1080
    scrcpy --new-display=1920x1080/420  # force 420 dpi
    scrcpy --new-display         # use the main display size and density
    scrcpy --new-display -m1920  # scaled to fit a max size of 1920
    scrcpy --new-display=/240    # use the main display size and 240 dpi

Fixes #1887 <https://github.com/Genymobile/scrcpy/issues/1887>
PR #5370 <https://github.com/Genymobile/scrcpy/pull/5370>

Co-authored-by: Simon Chan <1330321+yume-chan@users.noreply.github.com>
Co-authored-by: anirudhb <anirudhb@users.noreply.github.com>