Fixes #1472. Per-vertex variable validation fixes.

Relaxs checks for per-vertex builtin variables. If the builtin
decoration is applied to a variable, then those checks now allow a level
of arraying on the variable before checking the type consistency.

* Allows arrays of variables to be present for the per-vertex variables:
 * Position
 * PointSize
 * ClipDistance
 * CullDistance
* Updated tests