The interface SurfaceTool presents for tangents and all its usages work
with tangent frames represented as 4 values: tangent direction and
scalar orientation (+1 or -1).
However, internally it was represented as tangent+bitangent (mistakenly
called binormal), which introduces additional conversions back & forth
and is problematic for replacing the tangent space generation algorithm.
This change converts the internal storage and adjusts the calling code
accordingly. Note that since SurfaceTool::set_tangent is exposed to
scripts, I've kept the Plane interface there even though it's misleading
to use this type here.
Arseny Kapoulkine
d00e574657