Better enum type support for Mesh types

[jespertheend]

Mesh.js currently contains:
https://github.com/rendajs/Renda/blob/b8fbb5998d142f8c8c0dfc9ba8aad256414c3c95/src/core/Mesh.js#L5-L8
so functions like Mesh.getBufferForAttributeType() use unclear arguments such as:

mesh.getBufferForAttributeType(0);

We need an enum for this but we need to make sure it still serializes properly through binarySerialization.js and with closure compiler.