When map is tilted, flat annotation views should be skewed

[1ec5]

Both the user dot/puck and the flat annotation views introduced in #5067 rely on CATransform3DRotate() to rotate the annotation view in 3D space. But the transform is based on CATransform3DIdentity, so the annotations appear to be squashed, whereas the map is skewed with a perspective transform:

    Untilted

    Tilted

Notice how the left and right edges of each annotation remain perpendicular to the top and bottom edges, even as the north-south roads come closer together towards the top of the view. (This is the same area depicted in #1266, by the way.)

/cc @boundsj @friedbunny

[1ec5]

http://stackoverflow.com/questions/6887786/understanding-catransform3d
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreAnimation_guide/AdvancedAnimationTricks/AdvancedAnimationTricks.html#//apple_ref/doc/uid/TP40004514-CH8-SW13

[1ec5]

#5245 isn’t going to make iOS SDK v3.3.0, so we need to remove the flat property from MGLAnnotationView, because that property doesn’t produce the intended, documented effect. It isn’t such a big deal with the user dot/puck, which is round and relatively small, so we won’t touch that for v3.3.0. We’ll leave this bug open to track the real fix in v3.4.0.

[1ec5]

#5546 backs out the flat property, pending #5245.