Rebind tuple to its destructured base type

I have an array of matrices where the matrix type is defined as

typealias Vec4 = (Float, Float, Float, Float)
typealias Mat4 = (Vec4, Vec4, Vec4, Vec4)

let matrices:[Mat4]

I need to send them to an OpenGL API that accepts a buffer of Floats. How
do I do this? withMemoryRebound(to:_:) requires the source type and the
destination type to be the same size,, while each Mat4 is 16 times the size
of Float.