Add a type field to the data provided for a vertex attribute

[afd]

Right now all vertex attributes are assumed to have 32-bit floating point type, but it would be good (and easy) to be more general here. We should add a TYPE or ELEMENT_TYPE field to the data provided for a vertex buffer. Right now there could be just one option - float. But this would pave the way for more options without having to make float the default.