In our current code, we would use a staging texture as the working
copy for buffer updates. This would trigger very large memcpy calls
in some cases for large buffers with small updates. Instead, use
a CPU memory buffer storage, and work with this storage when the
user updates data. This plays much nicer with the VertexDataManager.

BUG=angle:912

Change-Id: I8c32d3d9bb321a06534556ce05b4b99dc3d1e961
Reviewed-on: https://chromium-review.googlesource.com/249183Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>