Add a Vector2 type for storing texcoords and improve the sample by having
it draw several thousand instances instead of two.

BUG=angle:521

Change-Id: I87996a90000a83bc66d091c7537ea411080d7e9f
Reviewed-on: https://chromium-review.googlesource.com/188723Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>