Render groups footprint in an offscreen buffer and cache it for optimization

The current footprint rendering is done in real time for each group.
It should be possible to pre-render each group footprint in an offscreen buffer (e.g. in a texture using openGL extension) and just display this image when needed. Storing the images in a cache should allow very important speedup, and allow much improved rendering of more complex data. E.g. use footprint opacity to reflect quantities such as cumulated SNR, spatial resolution etc..
Precomputing the rendering for a footprint implies distortions problems when the displayed distortion is not the same as the precomputed one. This will be more visible on large footprints (or very distorted projection mode). The solution to this is to tesselate the image at display time. This particular task is the subject of another Stellarium Blueprint. See TODO