[darwin] Use explicit initialization rather than global static variables for the default RunLoop

When linking Mapbox GL as a static library, the object file (MGLFoundation.o) containing the global static variable initialization was not linked into the resulting binary since it was not referenced. This resulted in the RunLoop not being initialized in the main thread.

Reverts 03a14ff000

platform/darwin/src/MGLFoundation.mm

@@ -1,4 +1,6 @@
 #import "MGLFoundation_Private.h"
 
 /// Initializes the run loop shim that lives on the main thread.
-mbgl::util::RunLoop mgl_runLoop;
+void MGLInitializeRunLoop() {
+    static mbgl::util::RunLoop runLoop;
+}

platform/darwin/src/MGLFoundation_Private.h

@@ -2,4 +2,4 @@
 
 #include <mbgl/util/run_loop.hpp>
 
-extern mbgl::util::RunLoop mgl_runLoop;
+void MGLInitializeRunLoop();

platform/darwin/src/MGLOfflineStorage.mm

@@ -1,5 +1,6 @@
 #import "MGLOfflineStorage_Private.h"
 
+#import "MGLFoundation_Private.h"
 #import "MGLAccountManager_Private.h"
 #import "MGLGeometry_Private.h"
 #import "MGLNetworkConfiguration.h"
@@ -170,6 +171,8 @@ NSString * const MGLOfflinePackMaximumCountUserInfoKey = MGLOfflinePackUserInfoK
 }
 
 - (instancetype)init {
+    MGLInitializeRunLoop();
+
     if (self = [super init]) {
         NSURL *cacheURL = [[self class] cacheURLIncludingSubdirectory:YES];
         NSString *cachePath = cacheURL.path ?: @"";