Added support for DDS files. Have been really enjoying JPEGView but it's not able to open .dds files which are common in game development workflows.

Closes #326.

Overview

• I tried to keep the implementation lightweight and chose not to include external libraries.
• I chose not to use DirectXTex for DDS support as I felt it would add too much bloat to the project.
• I instead adapted the Block Compression implementation BcnDecode.c from the Python Pillow project from here.

Limitations

• Since I didn't use DirectXTex, I don't think I cover 100% of DDS subformats, especially more niche ones. However, I used Pillow's DDS support as a guideline and manually ensured parity between JPEGView and all .dds files Pillow uses in its tests
  • I believe this should cover the vast majority of dds files

This is my first PR here - do let me know if I'm missing anything else that needs to be added when supporting additional file formats.

TODO

• Check if DDS file thumbnail previews work in windows explorer
• I'll try and add support for the dds variants listed here. Will probably use the test media provided here to validate my implementations.