Split varyings into input and output varyings in renderer This patch intends to split all vector<Varying> into two vectors to store input and output varyings separately in the renderer. This patch is a base of supporting program linking with geometry shader. Unlike the vertex shaders (their outputs are varyings) and fragment shaders (their inputs are varyings), the inputs and outputs of geometry shaders are all varyings, so we need two vector<Varying> to store them correctly. BUG=angleproject:1941 TEST=angle_end2end_tests Change-Id: Ic4b8343f2fc3df87b764c45f2ea7810e565f7bee Reviewed-on: https://chromium-review.googlesource.com/720617 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org>