Clean up VaryingPacking This patch intends to clean up some issues in VaryingPacking to implement geometry shader easiler. 1. Use emplace_back() instead of push_back() when necessary. 2. Remove unnecessary parameter in VaryingPacking::packUserVaryings(). 3. Remove the assignment of semanticIndex and only handle them in D3D11 back-ends. BUG=angleproject:1941 Change-Id: Ia09c07f01dc442ce95cb4984e4b768d0c79872c7 Reviewed-on: https://chromium-review.googlesource.com/1128576 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>