fpi-device: Allow driver to handle identify for certain variants

Rather than requiring a driver to implement a variant with
identify support, and another one without, ask the driver whether a
particular device supports identification.

libfprint/fp-device.c

@@ -507,6 +507,9 @@ fp_device_supports_identify (FpDevice *device)
 
   g_return_val_if_fail (FP_IS_DEVICE (device), FALSE);
 
+  if (cls->supports_identify != NULL)
+    return cls->supports_identify (device);
+
   return cls->identify != NULL;
 }
 
@@ -946,6 +949,13 @@ fp_device_identify (FpDevice           *device,
   if (g_task_return_error_if_cancelled (task))
     return;
 
+  if (!fp_device_supports_identify (device))
+    {
+      g_task_return_error (task,
+                           fpi_device_error_new (FP_DEVICE_ERROR_NOT_SUPPORTED));
+      return;
+    }
+
   if (!priv->is_open)
     {
       g_task_return_error (task,

libfprint/fpi-device.h

@@ -79,6 +79,7 @@ struct _FpIdEntry
  * @delete: Delete a print from the device
  * @cancel: Called on cancellation, this is a convenience to not need to handle
  *   the #GCancellable directly by using fpi_device_get_cancellable().
+ * @supports_identify: Whether identify operations are supported.
  *
  * NOTE: If your driver is image based, then you should subclass #FpImageDevice
  * instead. #FpImageDevice based drivers use a different way of interacting
@@ -129,6 +130,8 @@ struct _FpDeviceClass
   void (*delete)   (FpDevice * device);
 
   void (*cancel)   (FpDevice *device);
+
+  gboolean (*supports_identify) (FpDevice *device);
 };
 
 /**