Generate contour lines from raster-dem source

Cutting this ticket to start a conversation about whether we want to extend the raster-dem source to support contour line generation. I have not started exploring the feasibility of this from a performance standpoint.

I'm not sure how many users would be interested in having configurable contour lines, so if this is something you would use please 👍 this ticket 🙃 .

There is probably a way to do this that I haven't thought of, but the most straightforward method I can think of would be doing a precalculation/Framebuffer pass similar to the hillshade layer that calculates the contours and outputs a new texture with the contour lines encoded which is what is rendered to the map. Making sure the contours line up between tiles would probably be tricky...

cc @mapbox/cartography-cats

The issue names the raster-dem source, hillshade layer, Framebuffer pass, textures, and tile alignment, but no files or tests. Start by locating those rendering paths and assess feasibility and performance. Done would require an agreed approach and scope for configurable contour generation, including cross-tile alignment.