Frame rate drops slightly on the map view when scrolling with lots of takeoff markers visible

[ntamas]

Steps to reproduce:

1. Open a larger show file with 80-100 drones and make sure that the takeoff markers are visible on the map.
2. Hide the takeoff markers in the settings of the "Mission info" layer.
3. Drag the map around a little bit with the mouse. Notice that the scrolling is buttery smooth.
4. Turn the takeoff markers back on.
5. Drag the map around again; notice that the frame rate drops slightly. It is not a big deal, but it's a bit annoying, considering that drawing 80-100 triangles should not put too much of a strain on the CPU / GPU.

I believe that we are missing some trivial optimization in OpenLayers, like setting updateLayerWhileInteracting or some other similar prop tofalse. Let's look a bit into this to see whether there are any low-hanging fruits for optimization.