PIXMA: decode SCAN button interrupt on MG3200 series (buf[4])

- https://gitlab.com/sane-project/backends/-/work_items/856
- https://gitlab.com/sane-project/backends/-/merge_requests/933

Index: backend/pixma/pixma_mp150.c
--- backend/pixma/pixma_mp150.c.orig
+++ backend/pixma/pixma_mp150.c
@@ -1049,6 +1049,15 @@ handle_interrupt (pixma_t * s, int timeout)
       s->events = PIXMA_EV_BUTTON1 | (buf[0x13] & 0x0f);
     }
   }
+  else if (s->cfg->pid == MG3200_PID)
+  /* MG3200 series: SCAN is the only button reported; flag in buf[4]. */
+  {
+    if (buf[4] & 1)
+    {
+      /* start scan */
+      s->events = PIXMA_EV_BUTTON1;
+    }
+  }
   else
   /* button no. in buf[0]
    * original in buf[0]
