[PATCH] d3dx9: Implement D3DXComputeTangent().

[Matteo Bruni]

A rare occasion where I can quote myself recursively, so I'll do just
that: https://www.winehq.org/pipermail/wine-devel/2019-April/143293.html

While it would be nice to have some nice and detailed tests along the
lines of mesh.c:test_compute_normals(), I'll settle for the minimum
necessary to confirm that the arguments to D3DXComputeTangentFrameEx()
are correct (or otherwise, e.g. if that can't be tested for whatever
reason, a small comment in the code).