Improve multiple monitor detection on Linux with i3wm

[belden]

My laptop is connected via HDMI to an external monitor. My window
manager is i3wm.

ffscreencast incorrectly detects my built-in and external displays
as a single very large screen. This is because xdpyinfo does not
report on physical hardware:

$ xdpyinfo | grep '^screen #' -A 1
screen #0:
  dimensions:    6096x2160 pixels (1612x571 millimeters)

xrandr correctly detects that I have two displays attached:

$ xrandr | grep ' connected'
eDP-1 connected primary 2256x1504+0+0 (normal left inverted right x axis y axis) 285mm x 190mm
DP-4 connected 3840x2160+2256+0 (normal left inverted right x axis y axis) 697mm x 392mm

This commit ensures the external displays are correctly detected.

---

Before:

$ ./ffscreencast --dry
/usr/bin/ffmpeg \
  -hide_banner \
  -loglevel info \
  -thread_queue_size 512 \
  -f x11grab \
  -video_size 6096x2160 \  💥 too big
  -i ":0.0" \              💥 no offset
  -c:v libx264 \
  -crf 0 \
  -preset ultrafast \
  -threads 0 "/home/blyman/Desktop/Screencast 2024-07-11 at 10.48.17.mkv"

After:

$ ./ffscreencast --dry
Available devices:
[1] eDP-1 connected primary 2256x1504+0+0 (normal left inverted right x axis y axis) 285mm x 190mm
[2] DP-4 connected 3840x2160+2256+0 (normal left inverted right x axis y axis) 697mm x 392mm
Enter device number: 2
/usr/bin/ffmpeg \
  -hide_banner \
  -loglevel info \
  -thread_queue_size 512 \
  -f x11grab \
  -video_size 3840x2160 \  💥 correct size of DP-4
  -i ":0.0+2256,0" \       💥 correct offset of DP-4
  -c:v libx264 \
  -crf 0 \
  -preset ultrafast \
  -threads 0 "/home/blyman/Desktop/Screencast 2024\-07-11 at 10.48.57.mkv"