nx_vmvres.nx

nx_vmvres.nx -- efficient MV-RESIDUAL coding. After MV prediction (nx_vmvpred) most residual components are exactly 0 (neighbouring blocks share motion on a pan/head-pan), so paying the varint's fixed 5-bit length prefix on every component is wasteful. Code instead: [1 bit nonzero-flag]; only if nonzero, the signed varint. A 0 residual costs 1 bit instead of 5 -> MVP's magnitude win finally shows up in the total bitrate. Composes nx_ventropy. license_tier: ORIGINAL