Change of map view's frame size changes the minimum zoom scale

[hwiesmann]

Whenever the frame of a map view is changed the minimum zoom level is changed, too. Though there is no reason for doing so unconditionally.

The minimum zoom level should only be changed when the frame is larger than the largest possible size of a map with the currently given zoom level.

A solution of this bug is given in issue #249 although the tile of the issue might be misleading.