Minor cleanup to setIndexBuffer. This cleans up the point sprites dirtying logic into applyIndexBuffer. This series of small optimizations gives about a 15% improvement on the draw call benchmark for the D3D11 backend with the null driver. BUG=angleproject:1155 Change-Id: I210dd408d11ef5a0b58b0ad32d1255c787a55fba Reviewed-on: https://chromium-review.googlesource.com/666047 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>