NV21 input support

Eliminates an extra copy when encoding Android camera preview images.

Checkasm test by Janne Grunau.
ARM assembly with improvements from Janne Grunau.
x86: SSE and AVX implementations of plane_copy

Also remove the MMX2 implementation.
x86: SSSE3 and AVX2 implementations of plane_copy_swap

For NV21 input.