Add SSSE3 implementation for ipred_paeth

intra_pred_paeth_w4_8bpc_c: 561.6
intra_pred_paeth_w4_8bpc_ssse3: 49.2
intra_pred_paeth_w8_8bpc_c: 1475.8
intra_pred_paeth_w8_8bpc_ssse3: 103.0
intra_pred_paeth_w16_8bpc_c: 4697.8
intra_pred_paeth_w16_8bpc_ssse3: 279.0
intra_pred_paeth_w32_8bpc_c: 13245.1
intra_pred_paeth_w32_8bpc_ssse3: 614.7
intra_pred_paeth_w64_8bpc_c: 32638.9
intra_pred_paeth_w64_8bpc_ssse3: 1477.6