fixes #1675: fix initial annotation markers

[incanus]

This does two things:

1. It abstracts the pattern of checking if there are any stale annotation tiles (moving them to the caller in the process), then passing those back to the update routine to invalidate said annotation tiles.
2. It uses that routine additionally in response to the sprite store loading, which necessarily loads after the sprite, which is after the style, which is the only way that either runtime-provided marker imagery or the default style sprite marker imagery will work. It extends the Observer pattern already used in the style and the map context for decoupled communications.

[incanus]

Here's a simple diff to check the behavior:

diff --git a/ios/app/MBXViewController.mm b/ios/app/MBXViewController.mm
index d94e41b..e7548c6 100644
--- a/ios/app/MBXViewController.mm
+++ b/ios/app/MBXViewController.mm
@@ -82,7 +82,14 @@ static NSUInteger const kStyleVersion = 8;
                                                                              target:self
                                                                              action:@selector(locateUser)];

-    [self restoreState:nil];
+//    [self restoreState:nil];
+
+    self.mapView.debugActive = YES;
+
+    MGLPointAnnotation *point = [MGLPointAnnotation new];
+    point.coordinate = CLLocationCoordinate2DMake(45, -123);
+    [self.mapView addAnnotation:point];
+    [self.mapView setCenterCoordinate:point.coordinate zoomLevel:15 animated:NO];
 }

 - (void)saveState:(__unused NSNotification *)notification
@@ -354,6 +361,8 @@ static NSUInteger const kStyleVersion = 8;

 - (MGLAnnotationImage *)mapView:(MGLMapView * __nonnull)mapView imageForAnnotation:(id <MGLAnnotation> __nonnull)annotation
 {
+    if (annotation.title == nil) return [MGLAnnotationImage annotationImageWithImage:[UIImage imageNamed:@"Icon.png"] reuseIdentifier:@"foo"];
+
     NSString *title = [(MGLPointAnnotation *)annotation title];
     NSString *lastTwoCharacters = [title substringFromIndex:title.length - 2];

[incanus]

I should add that this updateAnnotationTiles() wholesale business is ripe for replacement as part of #1688, which should move to incremental parsing and rendering for annotations.

[incanus]

Mind giving this a pass @friedbunny @1ec5 in light of the First Steps guide? Working good for me.

[friedbunny]

Lookin' at this...

[friedbunny]

Works well, polylines and markers both show up immediately when drawn in viewDidLoad:. 👍