pan/bi: Prefer 'texture_index' to 'image_index'

They're not images. Making the executive decision to fix the notation.
Not canonical.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7081>
2 files changed