pan/bi: Implement txb

Also uses the 8.8 signed fixed-point format.

dEQP-GLES2.functional.shaders.texture_functions.fragment.texture2d_bias

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>