QtQuick3D instancing table layout

Currently, the instancing table is hardcoded to contain 20 floating point numbers per instance. This is not sufficient to support the proposed glTF instancing extension. Since the extension is still preliminary, it is too early to support it in 6.2, but we need to decide whether we need a flexible layout.

It is not equally urgent to make it possible to use tables with different layouts, but this also needs to be researched. Do we want to support it for all materials and extend the shader generation logic? Another option is to only support it for unshaded custom materials.