For TEX instructions use lambda=0. When sampling from texture unit K we

For TEX instructions use lambda=0.  When sampling from texture unit K we
were using the partial derivatives of texcoord[K] but the coordinate used
for texture sampling may be something totally different (and texcoord[K]
might not be a real texture coord at all).  Net result was a bogus LOD is
sometimes used, often resulting in using the smallest mipmap level (a
constant color).
Just use zero for now (undef LAMBDA_ZERO to override).
Plus, some additional debug code.