Add support for Apple KTX format

[Erik-White]

Would there be any interest in adding support for the Apple KTX format? It would be very useful, at least for us, to be able to use this directly in ImageSharp. I saw that a request for adding support for CgBI images (another internal Apple image format) had been accepted, and this is a very similar situation.

It is a supercompressed variant of KTX1 using LZFSE. Once decrompressed from LZFSE, the textures can be decompressed as a standard ASTC compressed KTX texture.

The main challenge would be incorporating an LZFSE decoder in ImageSharp.Textures. I have an implementation that could, with a little improvement, be incorporated in ImageSharp.Textures as I have done for ASTC. LZFSE is thankfully a lot simpler and the scope is therefore much smaller than ASTC.

The other challenge is that this is an internal Apple variant with no published spec (that I could find).

I have made a proof of concept implementation: Erik-White@163f828

Layout

[8 bytes]  Magic: AAPL\r\n\x1A\n  (0x4141504C 0D0A1A0A)
[chunks]   Sequence of tagged chunks until end of file

Each chunk has an 8-byte header:

[4 bytes]  Data size (little-endian uint32)
[4 bytes]  Identifier (ASCII, e.g. "HEAD", "LZFS", "FILL")
[N bytes]  Chunk data

HEAD chunk (84 bytes) contains texture metadata. Key fields at byte offsets within the
chunk data:

Offset	Size	Field
16	4	Internal pixel format (OpenGL enum, e.g. 0x93B0 = ASTC 4x4)
24	4	Width
28	4	Height
32	4	Depth
36	4	Number of array elements
40	4	Number of faces
44	4	Number of mipmap levels

LZFS chunk holds the texture payload. The first 4 bytes after the chunk header are skipped,
and the remainder is LZFSE-compressed ASTC block data.