libvpx: add a unit test for plane_add_noise.
In so doing this fixes a couple of bugs: vpx_plane_add_noise.c needed to subtract a clamp instead of add. And the assembly (mmx sse) had assumptions that parameters were continuous in memory which was not true. Change-Id: I76f2c43cf54bfc838eb2edf8a443eaaa7565d7b5
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