[ios] Simplify and (internally) document MGLPointRounded()

`UIScreen.nativeScale` is available beginning in iOS 8, so the conditional is no longer necessary.

platform/darwin/src/MGLGeometry.mm

@@ -98,7 +98,7 @@ CLLocationDirection MGLDirectionBetweenCoordinates(CLLocationCoordinate2D firstC
 
 CGPoint MGLPointRounded(CGPoint point) {
 #if TARGET_OS_IPHONE || TARGET_OS_SIMULATOR
-    CGFloat scaleFactor = [UIScreen instancesRespondToSelector:@selector(nativeScale)] ? [UIScreen mainScreen].nativeScale : [UIScreen mainScreen].scale;
+    CGFloat scaleFactor = [UIScreen mainScreen].nativeScale;
 #elif TARGET_OS_MAC
     CGFloat scaleFactor = [NSScreen mainScreen].backingScaleFactor;
 #endif

platform/darwin/src/MGLGeometry_Private.h

@@ -138,6 +138,9 @@ MGLRadianCoordinate2D MGLRadianCoordinateAtDistanceFacingDirection(MGLRadianCoor
  */
 CLLocationDirection MGLDirectionBetweenCoordinates(CLLocationCoordinate2D firstCoordinate, CLLocationCoordinate2D secondCoordinate);
 
+/**
+ Returns a point with coordinates rounded to the nearest logical pixel.
+ */
 CGPoint MGLPointRounded(CGPoint point);
 
 MGLMatrix4 MGLMatrix4Make(std::array<double, 16> mat);