This makes the API easy to use with internal shaders and pipelines.
This is useful for the implementation of masked color clear.

Also renames the serials as shader serials. This is more precise than
program serials.

Bug: angleproject:2455
Change-Id: Ie6247d1212ed4df856b561a5e9f16c0378202588
Reviewed-on: https://chromium-review.googlesource.com/1032857
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>