Improve vpx_blend_* functions.
- 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