Indicate modes that are not suitable for desktop

In the output of "displaymode d", add a "!" to modes where
CGDisplaymodeIsUsableForDesktopGUI returns false.

README.md

@@ -28,12 +28,15 @@ Display 0:
 2560 x 1600 @60.0Hz *
 1280 x 800 @60.0Hz
 2880 x 1800 @60.0Hz
+640 x 480 @60.0Hz !
 
 Display 1:
 800 x 600 @75.0Hz *
 ```
 
-where each row is the width x height in pixels, with an `*` to indicate the current mode.  Display 0 is always the main display.
+where each row is the width x height in pixels.  `*` indicates the current mode, and `!` indicates modes that are not usable for the desktop.
+
+Display 0 is always the main display.
 
 ## Other options
 

displaymode.c

@@ -162,7 +162,10 @@ void PrintMode(CGDisplayModeRef mode) {
     const size_t width = CGDisplayModeGetWidth(mode);
     const size_t height = CGDisplayModeGetHeight(mode);
     const double refresh_rate = CGDisplayModeGetRefreshRate(mode);
-    printf("%zu x %zu @%.1fHz", width, height, refresh_rate);
+    const bool usable_for_desktop =
+        CGDisplayModeIsUsableForDesktopGUI(mode);
+    printf("%zu x %zu @%.1fHz%s", width, height, refresh_rate,
+           usable_for_desktop ? "" : " !");
 }
 
 // Prints all display modes for the main display.  Returns 0 on success.