Etcpak

This release of etcpak brings the following changes:

- The build system has been changed to Meson.
- PNG images compression and decompression is now using zlib-ng, which significantly reduces the image load times.
- BC4, BC5, ETC2_R and ETC2_RG compression modes are now available. The compression algorithms in use were previously used to compress alpha channels in BC3 and ETC2_RGBA modes.
- The command line options have been revamped to make more sense.
- BC7 compression mode has been added. The base algorithm is Rich Geldreich's 4-mode bc7enc, which has been SIMD-ified (AVX2 and AVX512) for a ~2.6x speed increase. The results are bit-exact with the original implementation.
- Removed option to save alpha channel as a separate file.
- It is now possible to load and save DDS files.

etcpak 1.0 improves ETC2 compression speed by using heuristics to select block compression mode. It also adds compression and decompression support for ETC2 T and H blocks. Additionally, multithreaded benchmark mode is available.

etcpak 0.7 improves handling of command line parameters, adds DXT1/DXT5 support, and significantly speeds up compression and decompression times.