Remove global zoom limitation and split tile's zoom and viewport zoom

[noktoborus]

E.g. walkers can draw scaled z19 tiles on z21 even if source (like osm.org) not supported it.

This is convenient if different sources support different zoom and the way to support smooth zooming in future.

I made a stub partially covering my request but it is terrible solution: #101

[podusowski]

The idea seems sound, though I think I would start with a min/max pair (say struct ZoomLevels) which is returned from Tiles. Also, your PR is mixed with this click branch, which I don't really know what to do with it. It kind of breaks the idea of my_position, which would typically be a position obtained from the GPS or something and provided by the app. And if app wanted to jump somewhere, you'd call MapMemory::center_at (might want to rename to jump_to when I think about it now :)).

[podusowski]

Getting position under the mouse is a nice example though.

[noktoborus]

Also, your PR is mixed with this click branch,

I will drop zoom branch.

which I don't really know what to do with it. It kind of breaks the idea of my_position

Now no one provide GPS data, so I assume what click to set position is good idea.

I have reworked the click branch, could you take a look? :)

[podusowski]

Now no one provide GPS data

I'm thinking about adding it to the demo soon.

[podusowski]

This is all I see there.

[noktoborus]

This is all I see there.

sorry, I forgot to commit the new changes :)