ATOM-16854 Texture Settings dialog box crashes in the Editor (#5777)

Changes include:
-Fixed image preview issue which was trying to convert a compressed texture directly to QImage.
-Fixed texture resolution display for cubemap in texture setting editor.
-Sort the preset names in preset combo box
-Fixed a crash when showing IBLSkybox preset info in texture setting editor

Signed-off-by: Qing Tao <55564570+VickyAtAZ@users.noreply.github.com>

Gems/Atom/Asset/ImageProcessingAtom/Code/Source/Processing/ImageConvert.h

@@ -51,9 +51,6 @@ namespace ImageProcessingAtom
     //Converts the image to a RGBA8 format that can be displayed in a preview UI.
     IImageObjectPtr ConvertImageForPreview(IImageObjectPtr image);
 
-    //Combine image with alpha image if any and output as RGBA8
-    IImageObjectPtr MergeOutputImageForPreview(IImageObjectPtr image, IImageObjectPtr alphaImage);
-
     //get output image size and mip count based on the texture setting and preset setting
 
     //other helper functions