[Piglit] Testing for GL_ARB_draw_instanced

[Eric Anholt]

Here's testing for almost everything else I came up with from the spec.
The weaknesses I'd note are:

* We don't test whether one of the required shader extensions or GL3
  is actually present.

* We don't test for the non-presence of variables if optional
  extensions are not present.

* We don't test very large instance counts.

* We don't test interaction with non-GL3 (EXT_gpu_shader4 or
  NV_vertex_program4).

* We don't test other implied behavior of text like:

   "The command

        void DrawElementsInstancedARB(enum mode, sizei count, enum type,
                const void *indices, sizei primcount);

    behaves identically to DrawElements except that <primcount>
    instances of the set of elements are executed, and the value of
    <instanceID> advances for each iteration.  It has the same effect
    as:

        if (mode, count, or type is invalid )
            generate appropriate error
        else {
            for (int i = 0; i < primcount; i++) {
                instanceID = i;
                DrawElements(mode, count, type, indices, i);
            }
            instanceID = 0;
        }"

  We test that basic instancing-related rendering works, and we test
  the explicitly mentioned error behavior for "type" in the spec, but
  for example checking whether count or mode are invalid is not done.