[Feature] DDS image support

[henrik-andersson-sus]

Added support for displaying DDS images using the Pfim library.

[henrik-andersson-sus]

One improvement might be to create a helper file to move the hash sets IMG_EXTS and PFIM_EXTS but also the functions BitmapFromFile, BitmapFromStream and BitmapFromPfim since they are used from multiple classes, but I'm unsure of the file structure to know where to place such file.

[henrik-andersson-sus]

I've rebased the branch based on the refactoring i a023a92 which also solves my concern regarding the file structure 👍Now it is implemented as a separate ImageDecoder

[love-linger]

Avalonia also supports multiple pixel formats, see PixelFormats

public static class PixelFormats
{
    public static PixelFormat Rgb565 { get; } = new PixelFormat(PixelFormatEnum.Rgb565);
    public static PixelFormat Rgba8888 { get; } = new PixelFormat(PixelFormatEnum.Rgba8888);
    public static PixelFormat Rgba64 { get; } = new PixelFormat(PixelFormatEnum.Rgba64);
    public static PixelFormat Bgra8888 { get; } = new PixelFormat(PixelFormatEnum.Bgra8888);
    public static PixelFormat BlackWhite { get; } = new PixelFormat(PixelFormatEnum.BlackWhite);
    public static PixelFormat Gray2 { get; } = new PixelFormat(PixelFormatEnum.Gray2);
    public static PixelFormat Gray4 { get; } = new PixelFormat(PixelFormatEnum.Gray4);
    public static PixelFormat Gray8 { get; } = new PixelFormat(PixelFormatEnum.Gray8);
    public static PixelFormat Gray16 { get; } = new PixelFormat(PixelFormatEnum.Gray16);
    public static PixelFormat Gray32Float { get; } = new PixelFormat(PixelFormatEnum.Gray32Float);
    public static PixelFormat Rgb24 { get; } = new PixelFormat(PixelFormatEnum.Rgb24);
    public static PixelFormat Rgb32 { get; } = new PixelFormat(PixelFormatEnum.Rgb32);
    public static PixelFormat Bgr24 { get; } = new PixelFormat(PixelFormatEnum.Bgr24);
    public static PixelFormat Bgr32 { get; } = new PixelFormat(PixelFormatEnum.Bgr32);
    public static PixelFormat Bgr555 { get; } = new PixelFormat(PixelFormatEnum.Bgr555);
    public static PixelFormat Bgr565 { get; } = new PixelFormat(PixelFormatEnum.Bgr565);
}