Add ccst to list of boxes

[leo-barnes]

yes, the list of boxes look good to me. the only thing that i found to be missing was the ccst box that libavif writes out for animated images: https://github.com/AOMediaCodec/libavif/blob/85979a79983573c908958cd27197290cd19ea7b6/src/write.c#L3504

Originally posted by @vigneshvg in #225 (comment)

[leo-barnes]

Ah, on second thought this is kind of expected. We need a separate table for the boxes used for sequences, and ccst is part of that. Removing the blocking label.