vo/x11: read xrandr name (e.g., "LVDS1")

XRRGetOutputInfo contains a "name" element which corresponds to to the
display names given to the user by the "xrandr" command line
utility. Copy it into the xrandr_display struct. This will accessed
via property in future commits.
vo: add VOCTRL_GET_DISPLAY_NAMES

To get names of displays mpv window is on.
For example, with x11 vos, xrandr names LVDS1, HDMI1, etc.
vo/x11: implement VOCTRL_GET_DISPLAY_NAMES

use xrandr_display->name (obtained from XRROutputInfo->name)
command: add display-names property