Normal map (normal) layer type

[ibesora]

Adds a new layer type called normal that given a source layer with raster type pointing to a normal map pyramid, is shaded with the light defined on the style object.

I've created a new draw_normal.js where the webgl initialization is done and two files: normal.fragment.js and normal.vertex.js with the normal mapping shader. The remaining files are modified to update the spec to support the new type and its parameters (the same as a raster layer)

[mourner]

Is this similar to #4701 @kkaefer @mollymerp?

[anandthakker]

@ibesora thank you very much for submitting this PR!

As @mourner notes above, we are working on similar (but not identical) functionality over in #4701, with the hillshade layer type.

With that new type already being added to the spec, I think that the normal layer you've created here would be fit better as a custom layer. The custom layer API is still in early design/development, but I think this is a great use case for us to take into account while working on it!