We can use the format tables and input layout to determine when we need
to generate conversion code in the vertex shader.

BUG=angle:560
Change-Id: Ib64fb16823bf78ed6432fba283b0d24bcb673a76
Reviewed-on: https://chromium-review.googlesource.com/185195Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>