Generate code for buffer fallbacks as well as texture fallbacks.
No functional change.

BUG=angleproject:2405

Change-Id: I9f30a2cbb3cd9ba1d18474f99cba434b030b0232
Reviewed-on: https://chromium-review.googlesource.com/1113026
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>