[webgl-point] Add feature overlay support to webgl map renderer.

Modified the icon sprite example to show blue butterflies overlayed over the normal ones.
Slightly rearranged canvas map renderer for readability.
We already have the 2D and 3D contexts.
It is hard to understand what means `vectorContext`.
Since the class is called `Immediate`, it seems sensible to call the object `immediate`.
I can change the canvas renderer for consistency.