Add SDL_TouchDeviceType enum and SDL_GetTouchDeviceType(SDL_TouchID id).

Touch device types include SDL_TOUCH_DEVICE_DIRECT (a touch screen with window-relative coordinates for touches), SDL_TOUCH_DEVICE_INDIRECT_ABSOLUTE (a trackpad-style device with absolute device coordinates), and SDL_TOUCH_DEVICE_INDIRECT_RELATIVE (a trackpad-style device with screen cursor-relative coordinates).

Phone screens are an example of a direct device type. Mac trackpads are the indirect-absolute touch device type. The Apple TV remote is an indirect-relative touch device type.

src/core/linux/SDL_evdev.c

@@ -475,6 +475,7 @@ SDL_EVDEV_init_touchscreen(SDL_evdevlist_item* item)
     }
 
     ret = SDL_AddTouch(item->fd, /* I guess our fd is unique enough */
+        SDL_TOUCH_DEVICE_DIRECT,
         item->touchscreen_data->name);
     if (ret < 0) {
         SDL_free(item->touchscreen_data->slots);