This cl enables transform feedback in Context by calling
Program::applyTransformFeedback(). It also adds the code
required to compute vertex offsets between different draw
calls, when multiple successive draw calls write into the
same transform feedback buffer(s).

Change-Id: Ib3bc4bdd1308486642f233f425d0088a55cb1333
Reviewed-on: https://swiftshader-review.googlesource.com/5301Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>