Added VertexArray.cpp with a few
trivial function implementations.

Change-Id: I9ea0bf47b2c6a1f76392c5fc84837f1778946855
Reviewed-on: https://swiftshader-review.googlesource.com/2984Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>