Encoder: Defined structures for accessing MB header data

Instead of using hard coded address offsets,
defined structures to access MB Header for each MB type
This improves readability of the code

Also helps in maintaining required alignment for MV
MV elements need to be aligned to 2 byte boundary
Unaligned accesses result in undefined behavior

Bug: 33073518
Test: Tested with -fsanitize=alignment enabled on avcenc
Change-Id: I51c009b7deded76315a4bf2880444d21ae7d517f
5 files changed