Support a normalize option on the GeoTIFF source

[tschaub]

By default, we normalize values in GeoTIFF sources so that style expressions on the layer work with values between 0 and 1. This change makes it so the GeoTIFF source accepts a normalize option to control this behavior. If normalize is set to false, then the raw GeoTIFF values will be make available to style expressions on the layer.

The current implementation uses 8-bit values for normalized data and 32-bit otherwise. In the future, we may change this to support higher bit depth for normalized values as well. But this implementation detail is not exposed to users.

I haven't had any luck finding a CORS-friendly source of elevation data hosted as GeoTIFF. Both the AWS Terrain Tiles and Copernicus DEM data are not CORS-friendly. But here is a movie of a elevation tile from the Terrain Tiles with a CORS proxy in front:

Fixes #12834.

[mike-000]

AWS Terrain Tiles are CORS friendly as long as you use https://elevation-tiles-prod.s3.amazonaws.com/ (https://s3.amazonaws.com/elevation-tiles-prod/ is not CORS enabled)

https://codesandbox.io/s/webgl-tiles-forked-yx38uh?file=/main.js