[ios] Use transparent image for annotations backed by views

The current implementation of view backed annotations mostly relies
on the existing annotation model manager and the associated underlying
boost query implementations for annotation management. However, since
the representation of the model is a view, an image was not installed
for view backed annotations. Because the underlying annotation
management implementation is not entire comfortable working without
an image, it would report
`[INFO] {Worker}[Sprite]: Can't find sprite named 'default_marker'`
when annotation models without an image were encountered.

This updates the annotation adding logic in to create (or reuse) and
install a small, invisible image for each view backed annotation.

Author: boundsj

platform/ios/src/MGLMapView.mm

@@ -88,6 +88,10 @@ typedef NS_ENUM(NSUInteger, MGLUserTrackingState) {
 /// Reuse identifier and file name of the default point annotation image.
 static NSString * const MGLDefaultStyleMarkerSymbolName = @"default_marker";
 
+/// Reuse identifier and file name of the invisible point annotation image used
+/// by annotations that are visually backed by MGLAnnotationView objects
+static NSString * const MGLInvisibleStyleMarkerSymbolName = @"invisible_marker";
+
 /// Prefix that denotes a sprite installed by MGLMapView, to avoid collisions
 /// with style-defined sprites.
 NSString *const MGLAnnotationSpritePrefix = @"com.mapbox.sprites.";
@@ -2861,6 +2865,14 @@ - (void)addAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations
                     annotationView.annotation = annotation;
                     annotationView.center = [self convertCoordinate:annotation.coordinate toPointToView:self];
                     [newAnnotationViews addObject:annotationView];
+                  
+                    MGLAnnotationImage *annotationImage = self.invisibleAnnotationImage;
+                    symbolName = annotationImage.styleIconIdentifier;
+                    annotationImagesForAnnotation[annotationValue] = annotationImage;
+                    if ( ! self.annotationImagesByIdentifier[annotationImage.reuseIdentifier])
+                    {
+                        [self installAnnotationImage:annotationImage];
+                    }
                 }
             }
 
@@ -2897,22 +2909,21 @@ - (void)addAnnotations:(NS_ARRAY_OF(id <MGLAnnotation>) *)annotations
 
             MGLAnnotationTag annotationTag = _mbglMap->addAnnotation(mbgl::SymbolAnnotation {
                 MGLPointFromLocationCoordinate2D(annotation.coordinate),
-                symbolName.UTF8String ?: ""
+                symbolName.UTF8String
             });
 
             MGLAnnotationContext context;
             context.annotation = annotation;
             MGLAnnotationImage *annotationImage = annotationImagesForAnnotation[annotationValue];
+            context.imageReuseIdentifier = annotationImage.reuseIdentifier;
 
-            if (annotationImage) {
-                context.imageReuseIdentifier = annotationImage.reuseIdentifier;
-            }
             if (annotationView) {
                 context.annotationView = annotationView;
                 context.viewReuseIdentifier = annotationView.reuseIdentifier;
             }
 
             _annotationContextsByAnnotationTag[annotationTag] = context;
+
             if ([annotation isKindOfClass:[NSObject class]]) {
                 NSAssert(![annotation isKindOfClass:[MGLMultiPoint class]], @"Point annotation should not be MGLMultiPoint.");
                 [(NSObject *)annotation addObserver:self forKeyPath:@"coordinate" options:0 context:(void *)(NSUInteger)annotationTag];
@@ -2962,6 +2973,23 @@ - (MGLAnnotationImage *)defaultAnnotationImage
     return annotationImage;
 }
 
+- (MGLAnnotationImage *)invisibleAnnotationImage
+{
+    MGLAnnotationImage *annotationImage = [self dequeueReusableAnnotationImageWithIdentifier:MGLInvisibleStyleMarkerSymbolName];
+    
+    if (!annotationImage)
+    {
+        UIGraphicsBeginImageContext(CGSizeMake(1, 1));
+        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
+        UIGraphicsEndImageContext();
+        annotationImage = [MGLAnnotationImage annotationImageWithImage:image
+                                                                           reuseIdentifier:MGLInvisibleStyleMarkerSymbolName];
+        annotationImage.styleIconIdentifier = [MGLAnnotationSpritePrefix stringByAppendingString:annotationImage.reuseIdentifier];
+    }
+    
+    return annotationImage;
+}
+
 - (MGLAnnotationView *)annotationViewForAnnotation:(id<MGLAnnotation>)annotation
 {
     MGLAnnotationView *annotationView = [self.delegate mapView:self viewForAnnotation:annotation];