Handle tile boundaries for convolutional raster operations

Summary from chat with @kylebarron:

Hillshade and similar raster operations need neighboring pixels to compute each output pixel. Current examples are mostly per-pixel transforms so handling tile edges are not a major issue. Hillshading is different since slope/aspect at tile edges requires elevation values from adjacent tiles. Without "padded reads" or some form of neighbor backfilling visible grid lines will appear at the internal COG tile boundaries.

Example with visible grid lines:

https://github.com/user-attachments/assets/902c5f46-15b2-4450-b252-73f202add027

Example branch: https://github.com/developmentseed/deck.gl-raster/tree/feat-hillshade-example

Useful sources from MapLibre:

- DEMData says it handles backfilling from neighboring tiles:
[maplibre-gl-js/src/data/dem_data.ts#L24-L27](https://github.com/maplibre/maplibre-gl-js/blob/main/src/data/dem_data.ts#L24-L27)
- Initial border fill by duplicating edge pixels:
[maplibre-gl-js/src/data/dem_data.ts#L75-L92](https://github.com/maplibre/maplibre-gl-js/blob/main/src/data/dem_data.ts#L75-L92)
- The actual backfillBorder(...) method in Maplibre:
[maplibre-gl-js/src/data/dem_data.ts#L124-L151](https://github.com/maplibre/maplibre-gl-js/blob/main/src/data/dem_data.ts#L124-L151)
- Current MapLibre helper that calls tile.dem.backfillBorder(): https://github.com/maplibre/maplibre-gl-js/blob/main/src/tile/tile_manager_raster_dem.ts#L4-L42

Other useful resources:
- https://www.mapzen.com/blog/mapping-mountains/
- https://tangrams.github.io/terrain-demos/?url=styles/imhof.yaml#12.825/37.8589/-121.9371

Small note on the Tangram/Mapzen link, they seem useful mostly for terrain styling ideas but not necessarily for runtime tile-boundary handling. In the post there's a description on computing normals from heightmaps but also a note that the normal maps can be precomputed offline and served as separate tiles (which moves the neighborhood/edge handling/problem into the tile-generation pipeline).