Initialize glGen* call id args to zero

Initialize glGenTexture, glGenBuffer call id buffer items to zero before
calls. Otherwise it's not easy to check if the call succeeded or not.

Assert the rule in debug gl context.

Make TesselatingPathRendererTests use debug gl context. It exercises
some of the GenBuffers call sites.

Review URL: https://codereview.chromium.org/1514033002
3 files changed