multiple monitor detection

[Psionic K]

I was going to add multiple monitor support to the dslide package.  I
have to physical monitors active now.

However `display-monitor-attributes-list` returns the following:

    (((name . "combined screen") (geometry 0 0 6400 2160) (workarea
6400 2080 0 80)
      (mm-size 1693 572) (frames #<frame Emacs 0x3cca5d50>) (source .
"fallback")))

When running `xrandr`, xrandr is installed, so I'm not sure why the
source is fallback.  I restarted Emacs.  (30.0.91).

Xrandr's output more distinctly shows the two physical monitors:

    $ xrandr
    Screen 0: minimum 320 x 200, current 6400 x 2160, maximum 16384 x 16384
    eDP connected 2560x1440+3840+0 (normal left inverted right x axis
y axis) 309mm x 174mm
       2560x1440     60.01*+  48.01
       <snip>
    HDMI-A-0 connected primary 3840x2160+0+0 (normal left inverted
right x axis y axis) 600mm x 340mm
       3840x2160     60.00*+  50.00    59.94    30.00    30.00
25.00    24.00    29.97    23.98
       <snip>

I'm not sure why the fallback is being used by
`display-monitor-attribute-list'.  Xrandr is available to
`executible-find`.

My goal is to detect the physical monitor and allow the user to
display dslide presentations on the other physical monitor and switch
between them.  I'm presently unable to decide how to recognize the two
physical monitors without rolling my own solution on top of xrandr.