Refactor the generation of checkerboard textures since they're used by
this sample and MipMap2D.

BUG=angle:521

Change-Id: I1ba8bc4927a7944eb8efad0c8aa71eba4619e457
Reviewed-on: https://chromium-review.googlesource.com/188729Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>