XMFLOAT3X4 data type and load/store functions

[walbourn]

The XMFLOAT4X3 type is a row-major 4x4 matrix with the last column implicitly [0 0 0 1]. It is a compact way to represent affine transformations (translation, scale, rotation, etc.). It can't capture projective transformations, but this is often more than adequate for doing world->local->view transformations.

HLSL uses column-major storage of matrices by default, so typically you perform a transpose of a 4x4 matrix to store it in an XMMATRIX or XMFLOAT4X4 going from CPU to GPU. In the cases where you want a 4x3 column-major, you typically had to do the following:

XMMATRIX mT = XMMatrixTranspose(m);
XMStoreFloat4(reinterpret_cast<XMFLOAT4*>(...), mT.r[0]);
XMStoreFloat4(reinterpret_cast<XMFLOAT4*>(...), mT.r[1]);
XMStoreFloat4(reinterpret_cast<XMFLOAT4*>(...), mT.r[2]);

Or if you were able to use three XMVECTOR types in aligned constant buffers, you could make it:

XMMATRIX mT = XMMatrixTranspose(m);
cb.row[0] = mT.r[0];
cb.row[1] = mT.r[1];
cb.row[2] = mT.r[2];

As it turns out the DirectX Raytracing API also needs wants the transposed 12-float matrix form as well. Therefore, this change adds a XMFLOAT3X4 to simplify this usage.