recolour/m51510: reduce default search range to 1

Reducing the search range from 8 to 1 significantly reduces the time
spent recolouring.  The value of 1 seems to be a reasonable trade-off
between distortion and runtime.

tmc3/TMC3.cpp

@@ -708,7 +708,7 @@ ParseParameters(int argc, char* argv[], Parameters& params)
   (po::Section("Recolouring"))
 
   ("recolourSearchRange",
-    params.encoder.recolour.searchRange, 8,
+    params.encoder.recolour.searchRange, 1,
     "")
 
   ("recolourNumNeighboursFwd",