st/mesa: fix integer texture border color for some formats (v2)

st/mesa: fix integer texture border color for some formats (v2)

And the clear color too, though that may be an issue only with GL_RGB if it's
actually RGBA in the driver.

NOTE: This is a candidate for the stable branches.

Reviewed-by: Brian Paul <brianp@vmware.com>
v2: The types of st_translate_color parameters were changed to gl_color_union
    and pipe_color_union as per Brian's comment.