Fix wrong read_rgb call in from_rgb8_source

[seysn]

I saw that YUVBuffer::from_rgb_source and YUVBuffer::from_rgb8_source were very similar and had approximately the same time in benches. I believe this is a typo and the second one was supposed to call read_rgb8 instead of read_rgb.

By changing that, benches changes a lot :

Before

test convert_rgb8_to_yuv_512x512   ... bench:   1,282,626.91 ns/iter (+/- 255,071.37)
test convert_rgb_to_yuv_512x512    ... bench:   1,361,627.65 ns/iter (+/- 24,648.61)

After

test convert_rgb8_to_yuv_512x512   ... bench:     614,115.41 ns/iter (+/- 19,137.52)
test convert_rgb_to_yuv_512x512    ... bench:   1,235,944.20 ns/iter (+/- 173,681.31)

[ralfbiedert]

Thanks! Will look into this later today, CI apparently has deprecation issues.

[ralfbiedert]

This is in 0.7.2