Use ForCES_HDRL, TLV_HDRL, and ILV_HDRL instead of various sizeof's, to
make it clearer what certain tests ensure are true (as, in other cases,
the #defines are subtracted from values that should be, at that point,
ensured not to be less than the #define in question).
There's no guarantee that those values are aligned on the appropriate
boundary, nor is there a guarantee that accessing misaligned values will
behave as desired.
