Sort xviz- and custom-layers together by z-index

[MGraefe]

Previously only xviz layers where sorted by z-index. Now it's possible
to set the property zIndex on a custom layer and have it rendered
before a xviz layer.

[MGraefe]

NOTE: I couldn't actually test this code because I for my life cannot figure out how to integrate my custom build of streetscape.gl into my application, so it would be nice if someone could actually confirm it works as expected.

In case someone wants to help me with my build/include problem, I did the following steps:

• Build streetscape.gl (yarn install && yarn run build)
• Run yarn link in modules/core
• Go to my project clean project (no node_modules)
• Run yarn link @streetscape.gl/core && yarn install
• Run yarn run start-live
• I get errors in chrome console stating: Using deck.gl/core 7.0.4 and 7.0.5 simultaneously.

The package.json of my projects has the correct version for streetscape.gl specified (beta-12), I tried a lot of different things but I just cannot get it working correctly.

[twojtasz]

@MGraefe I think you may need to link all the streetscape packages. Can you see if that works and report back.

@Pessimistress may also have other ideas.

[Pessimistress]

layerList = layerList.concat(...)

[MGraefe]

I fixed the concat error. I also set the default zIndex of custom layers to 3 so custom layers are the 'last' ones by default as they where before this change. Only that it's now possible to set the property zIndex on a custom layer and have it sorted into a different place.