New VertexAttribute data types

Added int and unsigned int as possible
internal storage types for VertexAttribute
on top of the existing float type by using
a union in order to be able to store the new
VertexAttribute types in their native types.

Change-Id: I5a98aeded065095df6b44fa20f4c293ae230bc37
Reviewed-on: https://swiftshader-review.googlesource.com/2828
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>