Support for exporting aligned maps

[RishikesavanRamesh]

Can we export the aligned maps as a whole/group of few into a single, merged.pgm file?

[MichaelGrupp]

Hi, that could definitely be a useful extension. Right now, the app is focusing on visualization of the alignment process (with GPU-rendering) while exporting only the alignment pose for each map. Merging into a new single map file could be of course also useful.

This would be a new feature, which would require UI and some sort of CPU-based imaging/drawing lib for creating the merged layers (e.g. via tinyskia, cairo etc). So something in the direction of what Cartographer is doing in its submap_painter / ROS map export. I'll try to see what can be done in this directio, when I have time.