Increase chrow row alignment to 16 bytes.

Increase chrow row alignment to 16 bytes.

This is done by expanding luma row to 32-byte alignment, since
 there is currently a bunch of code that assumes that
 uv_stride == y_stride/2 (see, for example, vp8/common/postproc.c,
 common/reconinter.c, common/arm/neon/recon16x16mb_neon.asm,
 encoder/temporal_filter.c, and possibly others; I haven't done a
 full audit).
It also uses replaces the hardcoded border of 16 in a number of
 encoder buffers with VP8BORDERINPIXELS (currently 32), as the
 chroma rows start at an offset of border/2.
Together, these two changes have the nice advantage that simply
 dumping the frame memory as a contiguous blob produces a valid,
 if padded, image.

Change-Id: Iaf5ea722ae5c82d5daa50f6e2dade9de753f1003