Test drawArrays and drawElements with non-zero divisor

ANGLE_instanced_arrays spec redefines drawArrays and drawElements in the
terms of drawArraysOneInstance and drawElementsOneInstance, so vertex
attrib divisors should affect them too. Test this.

Includes some minor code cleanup as well.
Merge pull request #638 from NVIDIA/one-instance-test

Test drawArrays and drawElements with non-zero divisor