Add ol.render.IRender#drawAsync
Add ol.render.canvas.Immediate#drawAsync
Add ol.render.webgl.Immediate#drawAsync
Add ol.render.canvas.Replay#drawAsync
Use drawAsync in render box
Use drawAsync in canvas Immediate#drawFeature
Merge pull request #1473 from elemoine/vector-api-zindex

[vector-api] Make canvas immediate API support zIndex