This adds support for WEBGL_draw_instanced_base_vertex_base_instance and
WEBGL_multi_draw_instanced_base_vertex_base_instance via the DrawCall.drawRanges interface. It resolves #197.

This change lets you specify the first vertex and/or instance in your range on a per-draw basis, like this:

drawCall.drawRanges(
  [offset, numElements, numInstances, baseInstance, baseElement],
  ...
)

The multidraw+baseVertex+baseInstance extension calls will be used if they're available, since they cover the most use cases (and provide gl_DrawID). Then if baseVertices or baseInstances have been set the non-multidraw baseVertex+baseInstance extension calls will be used. If not, the existing draw call preference order (multidraw then non-multidraw) takes over.

I added an example at examples/multi-draw-base-instance.html:

let drawCall = app.createDrawCall(program, triangleArray)
  .drawRanges(
    [0, 3, 1, 2], // draw one instance starting at instance index 2
    [3, 6, 2, 1], // draw two instances starting at instance index 1
    [3, 6, 2], // draw two instances (implicitly starting at index 0) 
    [0, 3, 3], // draw three instances (implicitly starting at index 0)
  );

I tested this with the existing examples/multi-draw.html and examples/multi-draw-base-instance.html, and also spot checked the rest of the examples and didn't see anything broken. That said I didn't explicitly test the drawElementsInstancedBaseVertexBaseInstanceWEBGL and multiDrawElementsInstancedBaseVertexBaseInstanceWEBGL calls, instead I just made sure they conformed to the API.