Expose RasterTileExtractor::reproject_to_webmercator to Godot

[kb173]

The RasterTileExtractor offers a function to reproject any raster data into webmercator and save it on the disk, but it is not yet exposed to Godot. Now that RasterTileExtractor::clip does no on-the-fly reprojection for performance reasons, it would make sense to have access to reproject_to_webmercator. Thus, it can be used for an on-demand preprocessing step to get everything into the same projection.

[chrisgraf]

the target projection should be flexible as we currently plan to use MGI Austria Lambert, but this could change regarding available geodata, region, purpose

[kb173]

We should consider adding a general reproject functionality. However, it needs to be well documented that it is not suitable for real-time use, as reprojecting can take minutes or perhaps even hours!