Use adaptive filter for full-color PNGs

vips/vips.c

@@ -647,7 +647,7 @@ vips_pngsave_go(VipsImage *in, void **buf, size_t *len, int interlace, int quant
   if (!quantize)
     return vips_pngsave_buffer(
       in, buf, len,
-      "filter", VIPS_FOREIGN_PNG_FILTER_NONE,
+      "filter", VIPS_FOREIGN_PNG_FILTER_ALL,
       "interlace", interlace,
       NULL
     );