ipred_paeth

Benchmarks:
- Kendryte K230:
intra_pred_paeth_w4_8bpc_c:       412.9 ( 1.00x)
intra_pred_paeth_w4_8bpc_rvv:     688.0 ( 0.60x)
intra_pred_paeth_w8_8bpc_c:      1206.6 ( 1.00x)
intra_pred_paeth_w8_8bpc_rvv:    1094.3 ( 1.10x)
intra_pred_paeth_w16_8bpc_c:     3889.7 ( 1.00x)
intra_pred_paeth_w16_8bpc_rvv:   1796.7 ( 2.16x)
intra_pred_paeth_w32_8bpc_c:     9797.2 ( 1.00x)
intra_pred_paeth_w32_8bpc_rvv:   4323.9 ( 2.27x)
intra_pred_paeth_w64_8bpc_c:    24242.5 ( 1.00x)
intra_pred_paeth_w64_8bpc_rvv:  10739.8 ( 2.26x)

- Banana Pi BPI-F3
intra_pred_paeth_w4_8bpc_c:       395.1 ( 1.00x)
intra_pred_paeth_w4_8bpc_rvv:     705.4 ( 0.56x)
intra_pred_paeth_w8_8bpc_c:      1184.9 ( 1.00x)
intra_pred_paeth_w8_8bpc_rvv:    1125.3 ( 1.05x)
intra_pred_paeth_w16_8bpc_c:     3807.8 ( 1.00x)
intra_pred_paeth_w16_8bpc_rvv:   1850.8 ( 2.06x)
intra_pred_paeth_w32_8bpc_c:     9985.1 ( 1.00x)
intra_pred_paeth_w32_8bpc_rvv:   2235.5 ( 4.47x)
intra_pred_paeth_w64_8bpc_c:    24040.4 ( 1.00x)
intra_pred_paeth_w64_8bpc_rvv:   5450.0 ( 4.41x)