title: glVertexAttrib*: which attribute indices are predefined?
body: <p>the glVertexAttrib family of functions allows to add generic attributes to each vertex.
  You can set a index that the data will be associated with. However, you can't choose the index arbitrarily, since I discovered that using indices lower than 4 will break standard (Normal/TexCoord) attributes, plus 0 is the vertex position.
  How can I find out, which will be the first real free index to use for custom attributes?</p>