Reduced the size of the struct by 8 bytes, which would be
a memory savings of 64800 bytes for 1080 resolutions.  Had
an extra byte, so created an is_4x4 for B_PRED or SPLITMV
modes.  This simplified the mode checks in
vp8_reset_mb_tokens_context and vp8_decode_mb_tokens.

Change-Id: Ibec27784139abdc34d4d01f73c09f43e9e10e0f5