Allow mat2/3/4 as attribute types #1652

That would be nice (and would help our WebGPU support in Babylon.js) to allow having attributes of types mat2/3/4 in the vertex shader.

According to @kainino0x, Vulkan does allow it, for Metal/D3D12 it has to be looked for.

Note that we can do it in WebGL by "binding" 4 vec4 in sequence with enableVertexAttribArray/vertexAttribPointer.