Set VertexReader's vtype_ to prevent oddness

This was causing debug builds never to render textures correctly, and I think it was causing issues on some mobile devices.  Hard to say what/when and could cause some of the strange regressions from the texture cache changes potentially.

See also 6b3ddae5545fe64262b907bb08941b599b50023d, vtype_ wasn't used before this commit AFAICT.

-[Unknown]