fix an off-by-one error in mapim

TODO

@@ -1,12 +1,6 @@
 - get some brightly coloured spots with nohalo / vsqbs on wobble.ws ... very
   odd, the interpolation shouldn't change between bands
 
-- try:
-
-	$ ./test_resample.py TestResample.test_mapim
-	** VIPS:ERROR:interpolate.c:518:vips_interpolate_bilinear_interpolate: assertion failed: ((int) x + 1 < VIPS_RECT_RIGHT( &in->valid ))
-	Aborted (core dumped)
-
 - still not happy about float->int mask conversion in im_vips2mask.c
 
 - looks like we have a race in tiled threadcache? see 

libvips/resample/mapim.c

@@ -182,9 +182,9 @@ vips_mapim_region_minmax( VipsRegion *region, VipsRect *r, VipsRect *bounds )
 		TYPE py = p1[1]; \
 		\
 		if( px < 0 || \
-			px > resample->in->Xsize || \
+			px >= resample->in->Xsize || \
 			py < 0 || \
-			py > resample->in->Ysize ) { \
+			py >= resample->in->Ysize ) { \
 			for( z = 0; z < ps; z++ )  \
 				q[z] = 0; \
 		} \