Before this patch, index buffers where always mapped in case we needed the
index data for the indexed point sprites workaround.
This patch makes it so the index buffer is only mapped in this case, when
we need to stream index data or when we need to fill the static copies of
the index buffers.

This make the memory usage of
http://alteredqualia.com/xg/examples/mammoth.html go down from 41MB to
28MB.

BUG=angleproject:516

Change-Id: I937506d06fd6f074ef2120469dbd235e20245fca
Reviewed-on: https://chromium-review.googlesource.com/283626Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>