Invalid result if textures coordinates are qual

[stefaniapedrazzi]

If multiple texture coordinates points to the exact same point the cotangentFrame function returns an invalid matrix.
This is because duv1 and duv2 will be zero vectors and will lead to a division by zero at this line:

glsl-perturb-normal/cotangent-frame.glsl

Line 16 in 1f8ec7f

float invmax = 1.0 / sqrt(max(dot(T,T), dot(B,B)));

In default texture mapping of meshes, the case where vertices points to the same texture coordinate is quite common.
Should not be cotangentFrame robust enough to handle these cases too?