Fix a bug with Canon LiDE 700 where bright areas overflow

On this scanner, white areas are easily seen to become yellow, implying
some kind of integer overflow in the blue channel first, followed by
others.

I can't be 100% sure this is the correct fix without knowing more about
the design or having a data sheet; this "target_code" was established
experimentally.

This is a fix to issue #43.

backend/genesys.c

@@ -3069,6 +3069,9 @@ genesys_send_shading_coefficient (Genesys_Device * dev)
           case CIS_CANONLIDE220:
             target_code = 0xf000;
             break;
+          case CIS_CANONLIDE700:
+            target_code = 0xc000; /* from experimentation */
+            break;
           default:
             target_code = 0xdc00;
           }