- Add unit tests for the API
- Update test expectations

+ Pass through point size from the vertex array to the shader,
required for new tests to pass.

BUG=angleproject:2306

Change-Id: Ib19436c1f4cb12873adea94f734c821363f9e27d
Reviewed-on: https://chromium-review.googlesource.com/1079993
Commit-Queue: Lingfeng Yang <lfy@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>