improve interpretation of maplut output

a mono image though a colour lut needs to take the lut's interpretation

ChangeLog

@@ -1,5 +1,6 @@
-8/10/14 started 7.40.10
+8/10/14 started 7.40.11
 - rework extra band handling for colour functions
+- set interpretation of matlut output more carefully
 
 8/9/14 started 7.40.10
 - icc_import and icc_transform checks the input profile for compatibility 

libvips/histogram/maplut.c

@@ -602,6 +602,13 @@ vips_maplut_build( VipsObject *object )
 	if( lut->Bands != 1 )
 		maplut->out->Bands = lut->Bands;
 
+	/* The Type comes from the image with many bands. A B_W index image,
+	 * for example, needs to become an RGB image when it goes through a
+	 * three-band LUT.
+	 */
+	if( lut->Bands != 1 )
+		maplut->out->Type = lut->Type;
+
 	g_signal_connect( in, "preeval", 
 		G_CALLBACK( vips_maplut_preeval ), maplut );
 	g_signal_connect( in, "posteval",