fixed a crash when the map is rotated more than 90 degrees

When -updateVisibleAnnotations is called with a map that has been rotated more than 90 degrees, `gridSize` is negative, causing an infinite loop. This leads to a large number of memory allocations which eventually ends in an out-of-memory crash.
Merge pull request #3 from arrtchiu/patch-1

fixed a crash when the map is rotated more than 90 degrees