gallium/auxiliary/vl: Fix blurry output of compute_shader_yuv

There is a linear sampler used, so add half texel offset
to avoid undesirable blur when input and output resolutions
are the same.

Reviewed-by: Thong Thai <thong.thai@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24272>