windowscodecs: Use predefined WICBitmapPaletteTypeFixedGray4 palette instead of a handmade one.

dlls/windowscodecs/converter.c

@@ -185,21 +185,19 @@ static HRESULT copypixels_to_32bppBGRA(struct FormatConverter *This, const WICRe
             IWICPalette *palette;
             UINT actualcolors;
 
-            if (source_format == format_2bppIndexed)
-            {
-                res = PaletteImpl_Create(&palette);
-                if (FAILED(res)) return res;
+            res = PaletteImpl_Create(&palette);
+            if (FAILED(res)) return res;
 
+            if (source_format == format_2bppIndexed)
                 res = IWICBitmapSource_CopyPalette(This->source, palette);
-                if (SUCCEEDED(res))
-                    res = IWICPalette_GetColors(palette, 4, colors, &actualcolors);
+            else
+                res = IWICPalette_InitializePredefined(palette, WICBitmapPaletteTypeFixedGray4, FALSE);
 
-                IWICPalette_Release(palette);
+            if (SUCCEEDED(res))
+                res = IWICPalette_GetColors(palette, 4, colors, &actualcolors);
 
-                if (FAILED(res)) return res;
-            }
-            else
-                make_grayscale_palette(colors, 4);
+            IWICPalette_Release(palette);
+            if (FAILED(res)) return res;
 
             srcstride = (prc->Width+3)/4;
             srcdatasize = srcstride * prc->Height;