improve accuracy of vector path

ChangeLog

@@ -5,6 +5,7 @@
 - configure test for g++ 7.2 and composite.cpp
 - don't Ping in magickload, too unreliable
 - ensure WebP can add metadata when compiled with libwebpmux [lovell]
+- improve accuracy of vector path convolution [felixbuenemann]
 
 12/2/18 started 8.6.3
 - use pkg-config to find libjpeg, if we can

libvips/convolution/convi.c

@@ -909,7 +909,7 @@ vips_convi_intize( VipsConvi *convi, VipsImage *M )
 	 * later, so 1.0 (for example) would become 128, which is outside
 	 * signed 8 bit. 
 	 */
-	shift = ceil( log2( mx + 1 ) );
+	shift = ceil( log2( mx ) + 1 );
 
 	/* We need to sum n_points, so we have to shift right before adding a
 	 * new value to make sure we have enough range.