Regression: ColorType is no longer persisted

[nonkeltjebob]

Magick.NET version

14.13.1

Environment (Operating system, version and so on)

q8, x64, Windows/Linux

Description

Hi,

When setting a color type to PaletteAlpha, it no longer seems to stick, while this was the case on 14.13 and earlier.

Steps to Reproduce

`using var img = new MagickImage("samples/synth_input.png").Clone();
img.SetAttribute("colorspace:auto-grayscale", "false");

var saved = img.ColorType; // PaletteAlpha
img.Thumbnail(new MagickGeometry(400, 400)); // upcasts to TrueColorAlpha on both versions
img.ColorType = saved; // 14.11.1: re-quantizes. 14.13.1: does nothing.

Console.WriteLine(img.TotalColors); // should be ≤ 256`

expected (v14.11.1)
img.TotalColors = 248 (<256, palette restored)
actual (v14.13.1)
img.TotalColors = 159857 (still truecolor; ColorType setter ignored)

Images

synth_input.png