This is first part of work on porting mapbox/mapbox-gl-js#8694 - in follow up patch(es) it is required to remove CPU side copy using 2d canvas support on all supported platforms, similar to approach taken in gl.js https://github.com/mapbox/mapbox-gl-js/pull/8694/files#diff-34dbe5f7de34dc4b9a8745dcde9bdc37R48
Decoding on CPU removed.
Padding is still done in DEMData() but, instead od doing it wwhile decoding, it is using memcpy to pad original values.
Rebase to latest mapbox-gl-js master and re-generate shaders.
Partly fixes: #15503
