camera: Detect `exclusive_caps` v4l2loopback devices (#16116)

src/core/linux/SDL_udev.c

@@ -485,6 +485,7 @@ static int device_class(struct udev_device *dev)
 {
     const char *subsystem;
     const char *val = NULL;
+    const char *attr = NULL;
     int devclass = 0;
 
     subsystem = _this->syms.udev_device_get_subsystem(dev);
@@ -496,8 +497,18 @@ static int device_class(struct udev_device *dev)
         devclass = SDL_UDEV_DEVICE_SOUND;
     } else if (SDL_strcmp(subsystem, "video4linux") == 0) {
         val = _this->syms.udev_device_get_property_value(dev, "ID_V4L_CAPABILITIES");
-        if (val && SDL_strcasestr(val, "capture")) {
-            devclass = SDL_UDEV_DEVICE_VIDEO_CAPTURE;
+        if (val) {
+            if (SDL_strcasestr(val, "capture")) {
+                devclass = SDL_UDEV_DEVICE_VIDEO_CAPTURE;
+            }
+            // v4l2loopback may report output caps before reporting capture caps
+            // and ID_V4L_CAPABILITIES is never updated
+            else if (SDL_strcasestr(val, "video_output")) {
+                attr = _this->syms.udev_device_get_sysattr_value(dev, "state");
+                if (attr && SDL_strcmp(attr, "capture") == 0) {
+                    devclass = SDL_UDEV_DEVICE_VIDEO_CAPTURE;
+                }
+            }
         }
     } else if (SDL_strcmp(subsystem, "input") == 0) {
         // udev rules reference: http://cgit.freedesktop.org/systemd/systemd/tree/src/udev/udev-builtin-input_id.c