windowscodecs: Fix handling of a tRNS PNG chunk in the frame CopyPalette method.

dlls/windowscodecs/pngformat.c

@@ -798,7 +798,7 @@ static HRESULT WINAPI PngDecoder_Frame_CopyPalette(IWICBitmapFrameDecode *iface,
     png_colorp png_palette;
     int num_palette;
     WICColor palette[256];
-    png_bytep trans;
+    png_bytep trans_alpha;
     int num_trans;
     png_color_16p trans_values;
     int i;
@@ -822,23 +822,18 @@ static HRESULT WINAPI PngDecoder_Frame_CopyPalette(IWICBitmapFrameDecode *iface,
         goto end;
     }
 
+    ret = ppng_get_tRNS(This->png_ptr, This->info_ptr, &trans_alpha, &num_trans, &trans_values);
+    if (!ret) num_trans = 0;
+
     for (i=0; i<num_palette; i++)
     {
-        palette[i] = (0xff000000|
+        BYTE alpha = (i < num_trans) ? trans_alpha[i] : 0xff;
+        palette[i] = (alpha << 24 |
                       png_palette[i].red << 16|
                       png_palette[i].green << 8|
                       png_palette[i].blue);
     }
 
-    ret = ppng_get_tRNS(This->png_ptr, This->info_ptr, &trans, &num_trans, &trans_values);
-    if (ret)
-    {
-        for (i=0; i<num_trans; i++)
-        {
-            palette[trans[i]] = 0x00000000;
-        }
-    }
-
 end:
 
     LeaveCriticalSection(&This->lock);

dlls/windowscodecs/tests/pngformat.c

@@ -577,9 +577,7 @@ static void test_png_palette(void)
     hr = IWICPalette_GetColors(palette, 256, color, &ret);
     ok(hr == S_OK, "GetColors error %#x\n", hr);
     ok(ret == count, "expected %u, got %u\n", count, ret);
-todo_wine
     ok(color[0] == 0xff010203, "expected 0xff010203, got %#x\n", color[0]);
-todo_wine
     ok(color[1] == 0x00040506, "expected 0x00040506, got %#x\n", color[1]);
 
     IWICPalette_Release(palette);