Overlays Drawing improvements

This is the first part of my refactoring of the overlays drawing.
It fixes anchor for markers, also allow specifying anchor directly (float between 0 and 1).

Now this is also important for the next step.
Today on every draw, the overlay item position on map is recomputed. It should only be done on zoom change and position change.
It would not prevent drawing from being ok as we only have to offset using the projection ScreenRect