Shadow for buildings

[Gustl22]

See #575.
Activate with BuildingLayer.SHADOW = true;

Troubleshooting:

• OffscreenRenderer not works together with ShadowRenderer yet. Colors have to be cleared correcltly and must be mixed in another way.
• ExtrusionRenderer uses the call ebs[i] = null; this must be removed to allow render buildings twice: once from light, once from users view; may even is not necessary (thanks to garbage collecting)
• Some hack implementation of setting the light MVP to current tile. See ExtrusionRenderer.setMatrix()

Have fun :)

[Gustl22]

Without	With shadow

[devemux86]

Exceptional result and you know that I am strict in PR reviews!

I like the #ifdef SHADOW for having versatile shaders.
Just reversed some if-else so that the fallback is on GL20.

Merged via b4a98ef.

[devemux86]

Can we keep here the null set as before, when not using shadows?

[Gustl22]

That needs an extra local variable. E.g. such as "mMultipleRenders". But is this really needed?

[devemux86]

In any case mShader is available and could use a check like in line:373?

[Gustl22]

Works, but not the cleanest solution :)

[Gustl22]

I tested with Android Studio Profiler and I couldn't notice any remarkable memory regression when holding ExtrusionBuckets in ebs. If really needed, we should null all not used entries of mExtrusionBucketSet in BuildingRenderer.update after new ExtrusionBuckets are assigned (use activeTile count to null the rest).

for(int i = activeTiles; i < mExtrusionBucketSet.length && mExtrusionBucketSet[i] != null; i++) {
    mExtrusionBucketSet[i] = null;
}

But may it's not even needed or even costs more performance to explicitly nulling them, e.g. tags of MapElement aren't nulled either.