Added paper-length option to prevent double feeds

There was an old TODO in the code which suggests creating an option for using
the paper_length field to detect double feeds in the ADF.  I've added it as
an option, but set the default to not use it since it might cause older
scanners to get confused.

backend/avision.c

@@ -5584,11 +5584,8 @@ set_window (Avision_Scanner* s)
     }
   }
   
-  /* Newer scanners can utilize this paper length to detect double feeds.
-     However some others (DM152) can get confused during media flush if it
-     is set? TODO: wire to an option, ... */
-  /* if (dev->inquiry_paper_length)
-     set_double (cmd.window.descriptor.paper_length, (int)((double)30.0*1200)); */
+  if (s->val[OPT_PAPERLEN].w)
+     set_double (cmd.window.descriptor.paper_length, (int)((double)30.0*1200));
 
   if ( !(dev->hw->feature_type & AV_FUJITSU) )
     { 
@@ -6560,6 +6557,19 @@ init_options (Avision_Scanner* s)
   s->val[OPT_NVRAM].s = malloc(s->opt[OPT_NVRAM].size);
   s->val[OPT_NVRAM].s[0] = 0;
   
+  /* paper_length */
+  s->opt[OPT_PAPERLEN].cap = SANE_CAP_SOFT_SELECT | SANE_CAP_SOFT_DETECT | SANE_CAP_ADVANCED;
+  if (!dev->inquiry_paper_length)
+    s->opt[OPT_PAPERLEN].cap |= SANE_CAP_INACTIVE;
+  s->opt[OPT_PAPERLEN].name  = "paper-length";
+  s->opt[OPT_PAPERLEN].title = "Use paper length";
+  s->opt[OPT_PAPERLEN].desc  = "Newer scanners can utilize this paper length to detect double feeds.  However some others (DM152) can get confused during media flush if it is set.";
+  s->opt[OPT_PAPERLEN].type  = SANE_TYPE_BOOL;
+  s->opt[OPT_PAPERLEN].unit  = SANE_UNIT_NONE;
+  s->opt[OPT_PAPERLEN].size = sizeof(SANE_Word);
+  s->opt[OPT_PAPERLEN].constraint_type = SANE_CONSTRAINT_NONE;
+  s->val[OPT_PAPERLEN].w     = SANE_FALSE;
+  
   return SANE_STATUS_GOOD;
 }
 
@@ -7911,6 +7921,7 @@ sane_control_option (SANE_Handle handle, SANE_Int option,
 	case OPT_MULTISAMPLE:
 	case OPT_QSCAN:
 	case OPT_QCALIB:
+	case OPT_PAPERLEN:
 	  *(SANE_Word*) val = s->val[option].w;
 	  return SANE_STATUS_GOOD;
 	  
@@ -7981,6 +7992,7 @@ sane_control_option (SANE_Handle handle, SANE_Int option,
 	case OPT_OVERSCAN_TOP:
 	case OPT_OVERSCAN_BOTTOM:
 	case OPT_BACKGROUND:
+	case OPT_PAPERLEN:
 	  s->val[option].w = *(SANE_Word*) val;
 	  return SANE_STATUS_GOOD;
 	  

backend/avision.h

@@ -293,6 +293,8 @@ enum Avision_Option
   OPT_MESSAGE,           /* optional message from the scanner display */  
   OPT_NVRAM,             /* retrieve NVRAM values as pretty printed text */
   
+  OPT_PAPERLEN,          /* Use paper_length field to detect double feeds */
+
   NUM_OPTIONS            /* must come last */
 };