As in the aligned case, we can use VLSE64.V, though the way of doing so
gets more convoluted, so the performance gains are more modest:
get_pixels_unaligned_c: 126.7
get_pixels_unaligned_rvv_i32: 145.5 (before)
get_pixels_unaligned_rvv_i64: 62.2 (after)
For the reference, those are the aligned benchmarks (unchanged) on the
same T-Head C908 hardware:
get_pixels_c: 126.7
get_pixels_rvi: 85.7
get_pixels_rvv_i64: 33.2
If the scan lines are aligned, we can load each row as a 64-bit value,
thus avoiding segmentation. And then we can factor the conversion or
subtraction.
In principle, the same optimisation should be possible for high depth,
but would require 128-bit elements, for which no FFmpeg CPU flag
exists.
