pixdesc: add function to return pixel format with swapped endiannes
swscale: add support for endianness only conversion

Use bitfields in FormatEntry array to avoid wasting an int for each flag.
swscale: support endianness conversion for AV_PIX_FMT_XYZ12
vf_scale: add endianness conversion pixel format in query_formats
fate-jpeg2000-dcinema: force output to xyz12le

Fixes Fate tests on big-endian configs.