Wrong compression method in bmp header

[wh201906]

The data at offset 0x1E should be the compression method rather than the channel number

blurhash_ffi/lib/blurhash.dart

Line 555 in c249867

header.setUint32(0x1e, channels, Endian.little);

https://en.wikipedia.org/wiki/BMP_file_format#DIB_header_(bitmap_information_header)