Add RasterDemSource

[jgillich]

Description

Adds the RasterDemSource, which is used by HillshadeLayer. Currently Android-only because I don't own a Mac PC.

Test plan

Confirmed working with this snippet (it renders hills around Austria)

      HillshadeLayer(
        id = "hillshade123",
        source =
          rememberRasterDemSource(
            "https://demotiles.maplibre.org/terrain-tiles/tiles.json",
            tileSize = 256,
          ),
      )

Checklist

To your knowledge, are you making any breaking changes?

No

Have you tested the changes? On which platforms?

• Android: 16

[westnordost]

Maybe at least use TODO("Not implemented") or similar for clarity, after all, a TODO produces a runtime exception if that code is reached, IIRC.

[westnordost]

The JS documentation would be here:

https://maplibre.org/maplibre-gl-js/docs/API/classes/RasterDEMTileSource/

[westnordost]

Here is the iOS documentation: https://maplibre.org/maplibre-native/ios/latest/documentation/maplibre/mlnrasterdemsource

[westnordost]

Raster DEM sources have the additional property encoding which is missing here. Could be represented with

/** The encoding used by a Raster DEM source. */
public sealed interface RasterDemEncoding {

  /** Mapbox Terrain RGB tiles. See https://www.mapbox.com/help/access-elevation-data/#mapbox-terrain-rgb for more info */
  public data object Mapbox : RasterDemEncoding

  /** Terrarium format PNG tiles. See https://aws.amazon.com/es/public-datasets/terrain/ for more info. */
  public data object Terrarium : RasterDemEncoding

  /** 
    Custom format using the given [redFactor], [blueFactor], [greenFactor] and [baseShift] parameters.
    Currently only supported in JS.
  */
  public data class Custom(
    /** Value that will be multiplied by the red channel value when decoding. */
    val redFactor: Float = 1f,
    /** Value that will be multiplied by the blue channel value when decoding. */
    val blueFactor: Float = 1f,
    /** Value that will be multiplied by the green channel value when decoding. */
    val greenFactor: Float = 1f,
    /** Value that will be added to the encoding mix when decoding. */
    val baseShift: Float = 0f,
  ) : RasterDemEncoding
}