[Survey transects generation] move some of the logic handling out of function that handles only one polygon

Fixed camera shot count when trigger distance == 0

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

Also don't save to settings

This prevents intersection misses on more cases

Fixed bugs the unit test found

* If new Survey is set to altitude fixed value then use mission default
altitude
* If the user applies a new altitude to the entire mission change
surveys to fixed altitude and apply altitude

Caused intersection failure in some cases