MiniMap rendering should be done using OpenGL

[SirVer]

This is a rendering improvement made possible through the removal of the software renderer.

Right now the MiniMap renderer draws the minimap pixel by pixel. We could draw it more flexible by reusing functionality from the terrain renderer and draw it using OpenGL. This might make it possible to draw it at different sizes too, though drawing and later resizing might still bet a better solution.

Should investigation show that it is not feasible to change this rendering step from using OpenGL, we should instead render into an SDL_Surface so that the direct pixel access functionality in our surface class can be killed eventually.

[TiborB (tiborb95)]

I noticed that the minimap slows down the fps significantly, so it would be nice if this issue was mitigated or eliminated at all :)

[kaputtnik (franku)]

The attached savegame slows down fps from avg 30 to avg 15 when the minimap get opened.

[GunChleoc (gunchleoc)]

Migrated to https://github.com/widelands/widelands/issues/1575