backend/avision: implement ADF first-sheet offset compensation

i1120 has another offset. When scanning in ADF multi-page mode (the
default), the very first sheet is moved 1.5mm down. That leads to 1.5mm
being cut off at the bottom.
This implements the (hopefully last) offset setting for multi-sheet ADF
scans.

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

backend/avision.h

@@ -239,6 +239,7 @@ typedef struct Avision_HWEntry {
 
   /* ADF offsets in mm */
   struct {
+    float first; /* offset difference first sheet */
     mm_offset front; /* front-only */
     struct {
       mm_offset front;