Re-land "Compute packed varyings in ProgramD3D only." Instead of storing varying information in the shader, use a temporary set when linking a D3D program. This also means we won't have to modify information in the Shader object when linking a D3D program. This completes the refactoring for PackedVaryings. Re-land with fix for missing init of PackedVarying::vertexOnly. BUG=angleproject:1123 Change-Id: If110809c3817d88b0370ac575d739d7385b067d9 Reviewed-on: https://chromium-review.googlesource.com/296731 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>