Survey Grid improvements

Two things that would be helpful with planning mapping missions:
1. Being able to define a large area you want to map, set the maximum flight time of the airframe, and have survey grid automatically break up the area into different flight segments with X overlap between flights. This would make planning large area's much simpler and you could simply save each flight as a different waypoint file. When you finish the first flight, land, change batteries, load the second waypoint file, and fly again. No guess work on exactly where you left off or how much overlap you need between projects.
2. Use terrain data to show what the minimum overlap would be. We had this issue recently when mapping very mountainous terrain. All the estimations for overlap and side-lap are based on the altitude of the home beacon, however, if mapping a mountainous area, the overlap and side lap at higher elevations than where the survey grid made the estimations will be considerably less and can effect the quality of the orthomosaic. If there could be a "Minimum overlap" indicator based on the elevation of the area you are mapping, it could help set the proper parameters. Or, even more complex, have it change the overlap timing based on the terrain? Side lap would be difficult to adjust, but, overlap could be simple.

No files, tests, or entry points are named. Start by locating the existing Survey Grid planning workflow and reviewing how maximum flight time, overlap, side-lap, and terrain elevation are represented; done requires an agreed scope and verifiable behavior for flight segmentation and terrain-based minimum-overlap guidance.

Written by the indexing model from the issue text.