Copy C palette when merging

Author: radarhere

Tests/test_image.py

@@ -1076,6 +1076,12 @@ def test_apply_transparency(self) -> None:
             assert im.palette is not None
             assert im.palette.colors[(27, 35, 6, 214)] == 24
 
+    def test_merge_pa(self) -> None:
+        p = hopper("P")
+        a = Image.new("L", p.size)
+        pa = Image.merge("PA", (p, a))
+        assert p.getpalette() == pa.getpalette()
+
     def test_constants(self) -> None:
         for enum in (
             Image.Transpose,

src/_imaging.c

@@ -2419,7 +2419,12 @@ _merge(PyObject *self, PyObject *args) {
         bands[3] = band3->image;
     }
 
-    return PyImagingNew(ImagingMerge(mode, bands));
+    Imaging imOut = ImagingMerge(mode, bands);
+    if (!imOut) {
+        return NULL;
+    }
+    ImagingCopyPalette(imOut, bands[0]);
+    return PyImagingNew(imOut);
 }
 
 static PyObject *