You can either have a fully dedicated core for codecs, or you can just put certain codec related operations (like DCT-related SIMD) in your main cores. Cryptographic acceleration tends to use the latter approach.
Video codecs usually come with GPUs, not CPUs. It is only on SoCs where this distinction is a more fuzzy.
On a GPU, you didn't have an option to interleave normal program stream with specialized partial-decoding instruction. You put encoded frame in and you get decoded frame back, the media engine was separate block from compute.
Though this is also changing; see Intel GuC firmware, which has (optionally) some decoding, encoding and processing based on compute.