Allow reuse of layer rendering code without creating a map

[tschaub]

This is part of an effort to make the layer rendering code a bit more reusable. For the most part, layer rendering methods are provided what they need in the frame state, and they do not need access to a map or a map renderer.

Previously, the layer renderer's image loading handler would call map.render() directly. Now layer renderers are observable and they fire a change event on image load (if otherwise ready).

The WebGL layer renderers can be refactored to be given an ol.webgl.Context instead of a map renderer. This would replace all calls to mapRenderer.getContext() and mapRenderer.getGL() within the layer renderers. However, the WebGL tile layer renderer is still tied to the map renderer through its use of the tile texture queue and cache. If someone was motivated to change this, the tile renderer could be passed the queue and cache instead of maintaining a reference to the map renderer to access them. Until then the WebGL layer renderers are a special case in that they need a reference to a map renderer.

[elemoine]

Good changes. Looking forward to reworking the WebGL renderer. Please merge.