Unpack the index data into line strips, which can result in a
dynamically number of indices. To this end use a scratch vector in
Renderer11 so we don't end up allocating memory every draw call.

BUG=angleproject:597

Change-Id: Id3270611023cb6d163cd024e078d30ba5556e2ad
Reviewed-on: https://chromium-review.googlesource.com/310500Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>