foreign memory images tagges as ANY

we weren't tagging foreign memory buffers as supporting ANY demand
style, so they were falling back to SMALLTILE

this broke any operation using SEQUENTIAL_UNBUFFERED plus foreign
buffers, eg. falsecolour, since UNBUF needs very strict seq access

libvips/iofuncs/image.c

@@ -937,6 +937,7 @@ vips_image_build( VipsObject *object )
 			image->Type = VIPS_INTERPRETATION_MULTIBAND;
 
 		image->dtype = VIPS_IMAGE_SETBUF_FOREIGN;
+		image->dhint = VIPS_DEMAND_STYLE_ANY;
 
 		break;