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/666047Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>