Added separate Validate* functions for CopyBuffers(), BindTexImage(),
ReleaseTexImage(), SwapInterval(), and BindAPI().
For ValidateBindTexImage() the textureObject is set by validation when
appropriate and then used in the core function to prevent replicating
the textureObject look-up code.

Bug: angleproject:798
Change-Id: Ia32076a10aa0b044c2bdc8e2e24dd36d002c5022
Reviewed-on: https://chromium-review.googlesource.com/1180090Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>