Why matrix can't be passed as mat4x4?

[ivnsch]

In https://sotrh.github.io/learn-wgpu/beginner/tutorial7-instancing/#the-instance-buffer, it would be good to explain why we need to define a slot for each vec4. It's especially confusing since the camera uses directly mat4x4 in the shader, so a bit more background would be helpful.

// A mat4 takes up 4 vertex slots as it is technically 4 vec4s. We need to define a slot
// for each vec4. We'll have to reassemble the mat4 in the shader.