put interpolate param back

Although no current interpolators use the interpolate param, having it
there makes it possible for other (perhaps user-defined) interpolators
to be parameterised.

For example, you could write:

	vips affine in.v out.v "2 0 0 2" --interpolate sharp[2]

and `sharp` could do a bicubic and sharpen by 2 in a single operation.

See https://github.com/jcupitt/libvips/pull/372

libvips/resample/affine.c

@@ -351,7 +351,8 @@ vips_affine_gen( VipsRegion *or, void *seq, void *a, void *b, gboolean *stop )
 					(int) iy - window_offset + 
 						window_size - 1 ) );
 
-				interpolate( NULL, q, ir, ix, iy );
+				interpolate( affine->interpolate, 
+					q, ir, ix, iy );
 			}
 			else {
 				for( z = 0; z < ps; z++ )