implement pan constraints

[incanus]

• Overall, the map view should be constrained to not show areas outside of the Mercator projection on screen (barring wraparound as done in Tile wrapping #14) the same way that it is zoom constrained.
• Then, there should be an API for setting those constraints to an area arbitrarily smaller than the whole world.

[incanus]

Part of this will also include auto-adjusting the minimum zoom level possible such that the entire viewport is filled with map.

@kkaefer I think this should go in at the C++ level so that we basically don't ever see this:

Or ever allow this to happen:

But if you disagree and see a use case for this, I can push it up to the iOS-specific level as we definitely don't want it there.

[incanus]

Per chat, 👍 from @kkaefer.

[incanus]

Then, there should be an API for setting those constraints to an area arbitrarily smaller than the whole world.

Carrying this over to a new, low priority ticket.

[incanus]

#236

[incanus]

Also tackled here was adjusting min zoom to fit viewport limits.