[ios] Avoid drawing view annotations across pixel boundaries

[friedbunny]

Fixes #10119 by rounding view anchor points so that annotation views are not being drawn across pixel boundaries. This change impacts how all view annotations are drawn, including the user location annotation, regular view annotations, and also callout views.

In most cases, the misalignment was being introduced by -convertCoordinate:toPointToView:, which converts from geographic coordinates to screen points, but with floating point precision. This PR leaves the precision of that method intact, but rounds the returned point value to the nearest pixel before we use it to place the view.

Note that MGLPointRounded() will round to pixels and not necessarily points — it’s fine to draw to logical pixels on scaled displays.

Also fixed is the misalignment of the scale bar labels — these should be crisper now, too.

/cc @1ec5 @frederoni @fabian-guerra @boundsj

[friedbunny]

macOS doesn’t have UIScreen, so I’ll have to replace that with something more Darwin-friendly.

[fabian-guerra]

How the annotation's location accuracy will be affected at different zoom levels?

[friedbunny]

How the annotation's location accuracy will be affected at different zoom levels?

@fabian-guerra There are two scenarios, but in both the accuracy should remain unchanged (as this is always sub-pixel rounding of a screen point, just after it was converted from a geographic coordinate).

1. A user tracking mode is enabled and the user location annotation remains fixed at a constant point on the screen. The original placement point is unaffected by zoom and gets rounded once.
2. User tracking is disabled or this is a regular view annotation — the relative position of the view annotation is recalculated from its coordinate whenever the map moves, in which case the new position is rounded to the nearest pixel.

In both cases, the original geographic coordinate remains untouched and only the screen point at which the view annotation is placed is modified. At most, the difference introduced by the rounding will be ~0.5 pixels, which does not affect accuracy and subtly improves rendering sharpness.

[fabian-guerra]

LGTM 👍🏼