A serial of zero is a valid texture serial but it was used by
RenderbufferStorage to denote a non-texture binding. Added a isTexture
method to Renderbuffers and RenderbufferStorage
has been updated to return -1 as the current texture serial to avoid
collisions.

BUG=angle:633

Change-Id: Ie27e4b185ac09b48b593ae0dea02aeff5950a44f
Reviewed-on: https://chromium-review.googlesource.com/200123Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>