Image warping when cropping at sub-pixels

This presents itself the same as #241 (closed). It was introduced in 51a503e2.

I noticed here https://code.videolan.org/videolan/libplacebo/-/blob/e4e096be9512103381dd9c15f8c9a2669edf22c8/src/renderer.c?page=2#L1808 that ref->img.rect now gets rounded before being used later in pass->img.rect if i == pass->src_ref. Adding back in the subpixel offset by dividing by stretch_x/y, like was changed in 8299ec70, visually fixes the issue as far as I can tell. I don't know if there should be other considerations before or after that point in the function though.