Decode raster-DEM tiles without browser color management (#8125)

* Decode raster-DEM tiles without browser color management

DEM tiles encode elevation in their RGB channels, so decoding must be
exact. Request the tile image with {colorSpaceConversion: 'none'} so the
browser does not color-manage it; otherwise, with full color management
(e.g. Firefox with gfx.color_management.mode = 1), the color space is
assumed to be sRGB and the sRGB->display profile transform shifts the
channels containing elevation data.

* Trim the call site comment and document imageBitmapOptions instead

* Fold the color space assertion into the existing tile url test

* Apply suggestion from @HarelM

---------

Co-authored-by: Harel M <harel.mazor@gmail.com>