Refactor and unify HLSL varying signatures. We would use different code for all of our input and ouput stages in HLSL (vertex output, GS input, GS output, PS input). This was causing an HLSL error and some bugs in certain transform feedback instances. Fix this by using a helper method to use compatible code for each of the stages. BUG=angle:667 Change-Id: Idada969a775eccb5788723e0c7fa3ade9cb2d9ec Reviewed-on: https://chromium-review.googlesource.com/203778 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>