Add utility for computing tangents

[prideout]

Per discussion with Mathias and Aaron, we'd like to add a static utility method to VertexBuffer that makes it easier for clients to supply a buffer for TANGENTS.

It could look like this:

/**
 * Convenience function that consumes normal vectors (and, optionally, tangent vectors)
 * and produces quaternions that can be passed into a TANGENTS buffer.
 *
 * The given target buffer must be preallocated with at least count * sizeof(math::quat) bytes.
 *
 * Normals are required but tangents are optional, in which case this function tries to generate
 * reasonable tangents. The given normals should be unit length.
 *
 * If supplied, the tangent vectors should be unit length and should be orthogonal to the normals.
 * The w component of the tangent is a sign (-1 or +1) indicating handedness of the basis.
 *
 * Note that some applications and file formats (e.g. Blender and glTF) use mikktspace, which
 * requires full topology information and therefore cannot be computed here. This function
 * exists for simple use cases only.
 */
static BufferDescriptor&& populateTangentQuaternions(BufferDescriptor&& targetBuffer,
        size_t count,
        const math::float3* normals,
        size_t normalsStride = 0,
        const math::float4* tangents = nullptr,
        size_t tangentsStride = 0);

[romainguy]

That makes sense. Note that we also have a public function to compute the quaternion tangent frame from a TBN matrix. It's in mat3.h.

quatf q = mat3f::packTangentFrame({tangent, bitangent, normal});

[bejado]

What algorithm will the function use to generate the tangents if only normals are provided?

[prideout]

We'll simply have a fixed random vector like (0,1,0) and cross the normal with it, a bit like this although we should probably check for when the random vector is too close to the normal, in which case we'd choose a different random vector.

[romainguy]

MeshAssimp.cpp does that (minus switching vector when too close to the normal)