add and use move_paper to command sets

- add move_paper to command sets
- define move_paper only for mustek_gt6816_sheetfed_command_set who
  needs it

backend/gt68xx_devices.c

@@ -85,7 +85,8 @@ static GT68xx_Command_Set mustek_gt6816_command_set = {
   gt68xx_generic_setup_scan,
   gt68xx_generic_set_afe,
   gt68xx_generic_set_exposure_time,
-  gt68xx_generic_get_id
+  gt68xx_generic_get_id,
+  /* gt68xx_generic_move_paper */ NULL
 };
 
 static GT68xx_Command_Set mustek_gt6816_sheetfed_command_set = {
@@ -124,7 +125,8 @@ static GT68xx_Command_Set mustek_gt6816_sheetfed_command_set = {
   gt68xx_generic_setup_scan,
   gt68xx_generic_set_afe,
   gt68xx_generic_set_exposure_time,
-  gt68xx_generic_get_id
+  gt68xx_generic_get_id,
+  gt68xx_generic_move_paper
 };
 
 static GT68xx_Command_Set mustek_gt6801_command_set = {
@@ -164,7 +166,8 @@ static GT68xx_Command_Set mustek_gt6801_command_set = {
   gt68xx_generic_setup_scan,
   gt68xx_generic_set_afe,
   gt68xx_generic_set_exposure_time,
-  gt68xx_generic_get_id
+  gt68xx_generic_get_id,
+  /* gt68xx_generic_move_paper */ NULL
 };
 
 static GT68xx_Command_Set plustek_gt6801_command_set = {
@@ -203,7 +206,8 @@ static GT68xx_Command_Set plustek_gt6801_command_set = {
   gt68xx_generic_setup_scan,
   gt68xx_generic_set_afe,
   /* set_exposure_time */ NULL,
-  gt68xx_generic_get_id
+  gt68xx_generic_get_id,
+  /* gt68xx_generic_move_paper */ NULL
 };
 
 static GT68xx_Model unknown_model = {

backend/gt68xx_low.h

@@ -389,6 +389,14 @@ struct GT68xx_Command_Set
 
   /** Get the vendor, product and some more ids from the scanner */
     SANE_Status (*get_id) (GT68xx_Device * dev);
+
+  /** Move the paper by the amount of y offset needed to reach scan area
+   *
+   * @param dev Device object.
+   * @param request scan request used to compute move to reach scan area
+   */
+    SANE_Status (*move_paper) (GT68xx_Device * dev,
+			       GT68xx_Scan_Request * request);
   /*@} */
 };