This cleans up some messy stuff from the emulated index buffers, which
were caching variables that didn't need to be cached.

Also add in missing error checks. This touches a lot of code.

Re-land with a fix for clang-win build.

BUG=angleproject:1327
BUG=angleproject:1310

Change-Id: I31ed81c7242782bef7c5f6cde2192552f7ff9403
Reviewed-on: https://chromium-review.googlesource.com/336052Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>