VertexAttribute objects should have a default size of 4.

Fixes dEQP-GLES3.functional.state_query.shader.vertex_attrib_size

Change-Id: I597d2095f7be087f35a4adfb01bb2a3ba5200701
Reviewed-on: https://swiftshader-review.googlesource.com/14928Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>