vid: implement monitor selection with instant cvar changes

vid: implement monitor selection with instant cvar changes

Adds cvars: vid_display, vid_info_displaycount

Prints the monitors detected at startup and on hotplug.

Requires SDL headers >= 2.0.18 at build time.
At runtime older versions should still work but may lack some functionality.

May not work properly or at all if vid_desktopfullscreen is 0.

May require vid_restart after hotplug events.

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Changes existing cvars vid_fullscreen, vid_desktopfullscreen, vid_resizable,
vid_borderless to apply instantly.

Also (for windowed mode) works around window managers that move the
window after SDL creates and centres it (typically when adding a
titlebar), so the crosshair is always at the centre of the monitor.

Adds some missing SDL_WINDOWEVENT enumerations.

Changes the GL viewport size retrieval from screen coordinates to
pixels, which is a prerequisite for "HIGHDPI" aka "Apple Retina"
support.

Fixes https://gitlab.com/xonotic/darkplaces/-/issues/385

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>