- Made source buffers pointers to const.
- Renamed vpx_blend_mask6b to vpx_blend_a64_mask. This is more
  indicative that the function does alpha blending. The 6, or 6b
  suffix was misleading, as the max mask value (64) does not fit into
  6 bits.
- Added VPX_BLEND_* macros to use when needing to blend scalars.
- Use VPX_BLEND_A256 in combine_interintra to be more explicit about
  the operation being done.
- Added versions of vpx_blend_a64_* which take 1D horizontal/vertical
  masks directly and apply them to all rows/columns
  (vpx_blend_a64_hmask and vpx_blend_a64_vmask). The SSE4.1 optimzied
  horizontal version now falls back on the 2D version. This can be
  improved upon if it show up high enough in a profile.
- All vpx_blend_a64_* functions now support block sizes down to 1x1
  (ie: a single pixel). This is for usage convenience. The SSE4.1
  optimized versions fall back on the C implementation if
  w <= 2 or h <= 2. This can again be improved if it becomes hot code.

Change-Id: I13ab3835146ffafe3e1d74d8e9cf64a5abe4144d