VImage::ifthenelse() fix

the prototype was missing =0 on options for scalar constant forms

ChangeLog

@@ -21,6 +21,7 @@
   combine many images in bounded memory
 - VImage::write() implementation was missing
 - VImage::write() return value changed from void to VImage to help chaining
+- VImage::ifthenelse() with double args was missing =0 on options
 
 18/5/16 started 8.3.2
 - more robust vips image reading

cplusplus/include/vips/VImage8.h

@@ -751,19 +751,19 @@ public:
 	}
 
 	VImage 
-	ifthenelse( double th, VImage el, VOption *options )
+	ifthenelse( double th, VImage el, VOption *options = 0 )
 	{
 		return( ifthenelse( to_vector( th ), el, options ) ); 
 	}
 
 	VImage 
-	ifthenelse( VImage th, double el, VOption *options )
+	ifthenelse( VImage th, double el, VOption *options = 0 )
 	{
 		return( ifthenelse( th, to_vector( el ), options ) ); 
 	}
 
 	VImage 
-	ifthenelse( double th, double el, VOption *options )
+	ifthenelse( double th, double el, VOption *options = 0 )
 	{
 		return( ifthenelse( to_vector( th ), to_vector( el ), 
 			options ) );