improve the pixel rng

see https://github.com/libvips/libvips/issues/2642

ChangeLog

@@ -3,6 +3,7 @@
 - add support for another quantiser [DarthSim]
 - jpeg2000 load left-justifies bitdepth
 - add "password" option to pdfload
+- improve the pixel rng
 
 26/11/21 started 8.12.3
 - better arg checking for hist_find_ndim [travisbell]

libvips/iofuncs/util.c

@@ -1918,18 +1918,28 @@ vips_realpath( const char *path )
 /* A very simple random number generator. See:
  * http://isthe.com/chongo/tech/comp/fnv/#FNV-source
  */
+guint32 
+vips__random_add( guint32 hash, int value )
+{
+#define FNV_ADD( HASH, VALUE8 ) (((HASH) ^ (VALUE8)) * 16777619u)
+
+	hash = FNV_ADD( hash, value & 0xff );
+	hash = FNV_ADD( hash, (value >> 8) & 0xff );
+	hash = FNV_ADD( hash, (value >> 16) & 0xff );
+	hash = FNV_ADD( hash, (value >> 24) & 0xff );
+
+	return( hash ); 
+}
+
 guint32
 vips__random( guint32 seed )
 {
-	return( 1103515245u * seed + 12345 );
-}
+	guint32 hash;
 
-guint32 
-vips__random_add( guint32 seed, int value )
-{
-	seed = ((2166136261u ^ seed) * 16777619u) ^ value;
+	hash = 2166136261u;
+	hash = vips__random_add( hash, seed );
 
-	return( vips__random( seed ) ); 
+	return( hash );
 }
 
 static void *