Represent center_mode as Mercator projected coordinates?

[dwuertz]

While digging through the code, I got the impression that there is a lot of back-and-forth transformations between Mercator / screen coordinates and geodetic coordinates happening during each frame update. This is because, currently, the widget represents its internal state as geodetic coordinates, most prominently, MapMemory::center_mode. I propose to switch to normalized (i.e. zoom-independent) Mercator coordinates here instead. My expectation is that this would simplify the code overall and also improve performance / accuracy.

What do you think?

[podusowski]

It does sound interesting. I need to put some deeper thought though. I wonder how will it play with the far future plans that I have, namely 3d rendering of the globe, and more provider types, like WMS, which do not necessarily use the Mercator.

I also feel like these math code is generally duplicated between tile drawing functions and the Projector struct. On top of that, after your changes, we probably do not need the AdjustedPosition anymore. Basically, what I am trying to say here, is that I would like to do some of these first, and see how the code will look like afterwards.