Separate KTX and DDS fileformats into their own extension package BCnEncoder.Net.FileFormats

[Nominom]

Most users don't use ktx or dds capabilities at all, but instead use the raw-methods to do their encoding and decoding. By separating ktx and dds to an extension package, the default use-case becomes using the raw methods.

The extension package could also include support for new texture formats, like VTF.