When clearing an alpha-only format, set the alpha channel into red channel. Fixes `spec@ext_texture_integer@multisample-fast-clear gl_ext_texture_integer`. Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23497>