Support for GeoTIFF

Hi,

Thanks for this excellent plugin.

I have a single-band GeoTIFF file for elevation data. Would it be possible to fetch the file locally and get it to work with your module?
Please excuse my inexperience in this but I can't figure it out myself or if it is already possible with `tileFormat = dem` .

My thinking is to have the option for `arraybuffer` instead of `url` and/or `tileFormat = tiff` then modify the code for `_fetchTileData` to handle either `arraybuffer` directly or process through geotiff.js library like below.

```
fetch("file.tif")
.then(function (response) {
return response.arrayBuffer();
})
.then(function (arrayBuffer) {
return GeoTIFF.fromArrayBuffer(arrayBuffer);
})
.then(function (tiff) {
return tiff.getImage();
})
.then(function (image) {
const width = image.getWidth();
const height = image.getHeight();
const extent = image.getBoundingBox();
const raster = image.readRasters();
const rgbImage = image.readRGB();
})
```

Sorry about the code but I'm just listing the different values to get from geotiff.js.

Also, I found [this code in here](https://observablehq.com/@toja/shaded-relief) that can handle tiff files with an example file if it can be of help.