Make layers responsible for creating layer renderers

[tschaub]

Instead of having the map renderers depend on all layer types and all layer renderer types, the layers can take responsibility for creating layer renderers.

This will make it easier to create small builds without having to rely on @defines to indicate dependencies.

[tschaub]

There is a circular dependency here that is resolved by #6308. The vector tile renderer depends on an enum that is a property of the vector tile layer. And the vector tile layer depends on the vector rile renderer. After #6308, they both can depend on the render type enum without creating a circular dependency.

[ahocevar]

Very nice, thanks for taking this on! It is pretty much what I had in mind. Please merge after #6308.

[tschaub]

Rebasing now that #6308 is in.

[probins]

a good step forward, but it makes #6303 more difficult, doesn't it?

[tschaub]

@probins - we have two significant problems. One is that each of the map renderers depend on layer renderers for all types of layers. The other is that the map depends on both types of map renderer. The suggestion in #6303 would get rid of the ol.ENABLE_WEBGL flag, but we'd be left with the problem of depending on layer renderer types for all layer types. That is, we'd be left with ol.ENABLE_TILE, ol.ENABLE_IMAGE, ol.ENABLE_VECTOR, and ol.ENABLE_VECTOR_TILE.

I opted to get rid of four build flags first. That leaves us with one problem. We can pick up the discussion on #6303 - I'm not sure what the solution will be.