Hypsometric Tint from terrain-RGB tiles

[NathanMOlson]

Implements #5666 (Hypsometric Tint from terrain-RGB tiles). Supporting style spec changes are here: https://github.com/NathanMOlson/maplibre-style-spec/tree/color_relief

Demo here: https://nathanmolson.github.io/color_relief

This implements hypsometric tint in a similar way to raster and hillshade. The implementation in MapLibre feels clean to me, other than the duplication between drawRaster(), drawHillshade(), and drawColorRelief(). But they do things just differently enough that combining them doesn't seem fruitful.

The current rendering implementation resamples the colormap into a uniform interpolation table. This means that sharp breakpoints in the colormap are not captured accurately. I plan to benchmark two possible solutions: 1) Making the interpolation table larger (and finer). 2) Using the provided (non-uniformly sampled) colormap directly in the shader.

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• Write tests for all new functionality.
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