backend/avision: Rewrite offset compensation

This is a complete rewrite of the offset compensation using a
combination of overscan and line skipping. It replaces the option
AV_REAR_OFFSET by specifying exact offset values for each scanner.
This was needed for Kodak i1120 which needs bottom line skipping.
The old implementation could not skip bottom lines since we could
not know how many lines we will get in the end from ADF.
The new implementation redirects the output file descriptor to a
temporary file. After the current page is completely scanned,
offsets are applied and the output is passed to the original
output file descriptor.

Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>

backend/avision.h

@@ -81,6 +81,12 @@ typedef struct Avision_Connection {
 
 } Avision_Connection;
 
+/* structure for ADF offsets in mm */
+typedef struct mm_offset {
+  double top;
+  double bottom;
+} mm_offset;
+
 typedef struct Avision_HWEntry {
   const char* scsi_mfg;
   const char* scsi_model;
@@ -184,9 +190,6 @@ typedef struct Avision_HWEntry {
     /* does the scanner contain a Cancel button? */
   #define AV_CANCEL_BUTTON ((uint64_t)1<<28)
 
-    /* is the rear image offset? */
-  #define AV_REAR_OFFSET ((uint64_t)1<<29)
-
     /* some devices do not need a START_SCAN, even hang with it */
   #define AV_NO_START_SCAN ((uint64_t)1<<30)
 
@@ -234,6 +237,12 @@ typedef struct Avision_HWEntry {
     /* maybe more ...*/
   uint64_t feature_type;
 
+  /* ADF offsets in mm */
+  struct {
+    mm_offset front;
+    mm_offset rear;
+  } offset;
+
 } Avision_HWEntry;
 
 typedef enum {
@@ -328,6 +337,13 @@ enum Avision_Option
   NUM_OPTIONS            /* must come last */
 };
 
+/* structure for ADF offsets in pixels of HW res */
+typedef struct hwpx_offset {
+  int top;
+  int bottom;
+} hwpx_offset;
+
+
 typedef struct Avision_Dimensions
 {
   /* in dpi */
@@ -342,7 +358,11 @@ typedef struct Avision_Dimensions
 
   /* in pixels */
   int line_difference;
-  int rear_offset; /* in pixels of HW res */
+
+  struct {
+    hwpx_offset front;
+    hwpx_offset rear;
+  } offset;
 
   /* interlaced duplex scan */
   SANE_Bool interlaced_duplex;
@@ -434,6 +454,8 @@ typedef struct Avision_Device
   int inquiry_bits_per_channel;
   int inquiry_no_gray_modes;
 
+  SANE_Bool adf_offset_compensation;
+
   int scsi_buffer_size; /* nice to have SCSI buffer size */
   int read_stripe_size; /* stripes to be read at-a-time */
 
@@ -478,6 +500,7 @@ typedef struct Avision_Scanner
 
   /* Internal data for duplex scans */
   char duplex_rear_fname [PATH_MAX];
+  char duplex_offtmp_fname [PATH_MAX];
   SANE_Bool duplex_rear_valid;
 
   color_mode c_mode;