Support saving variable length rational TIFF tags

Tests/test_file_libtiff.py

@@ -355,6 +355,18 @@ def test_subifd(self, tmp_path: Path) -> None:
             # Should not segfault
             im.save(outfile)
 
+    def test_whitepoint_tag(
+        self, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+    ) -> None:
+        monkeypatch.setattr(TiffImagePlugin, "WRITE_LIBTIFF", True)
+
+        out = tmp_path / "temp.tif"
+        hopper().save(out, tiffinfo={318: (0.3127, 0.3289)})
+
+        with Image.open(out) as reloaded:
+            assert isinstance(reloaded, TiffImagePlugin.TiffImageFile)
+            assert reloaded.tag_v2[318] == pytest.approx((0.3127, 0.3289))
+
     def test_xmlpacket_tag(
         self, monkeypatch: pytest.MonkeyPatch, tmp_path: Path
     ) -> None:

src/encode.c

@@ -922,6 +922,18 @@ PyImaging_LibTiffEncoderNew(PyObject *self, PyObject *args) {
                     );
                     free(av);
                 }
+            } else if (type == TIFF_RATIONAL) {
+                FLOAT32 *av;
+                /* malloc check ok, calloc checks for overflow */
+                av = calloc(len, sizeof(FLOAT32));
+                if (av) {
+                    for (i = 0; i < len; i++) {
+                        av[i] = (FLOAT32)PyFloat_AsDouble(PyTuple_GetItem(value, i));
+                    }
+                    status =
+                        ImagingLibTiffSetField(&encoder->state, (ttag_t)key_int, av);
+                    free(av);
+                }
             }
         } else {
             if (type == TIFF_SHORT) {