Geometry Shaders: Don't require 'flat' for integer varyings. The 3.2 spec and the dEQP tests are quite clear in only requiring the 'flat' on fragment shader inputs. This restriction remains on older shader versions. Bug: angleproject:5409 Change-Id: Icc4b113b404280c2867a3e0bf6f77256a4316bff Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2574826 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>