Add runtime API for setting tile prefetch delta for Source class (#184)

* Add runtime API for setting tile prefetch delta for Source class

* Use Integer as param.

* Update gl native submodule.

MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/sources/Source.java

@@ -2,6 +2,8 @@ package com.mapbox.mapboxsdk.style.sources;
 
 import androidx.annotation.Keep;
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
 import com.mapbox.mapboxsdk.LibraryLoader;
 import com.mapbox.mapboxsdk.utils.ThreadUtils;
 
@@ -68,6 +70,28 @@ public abstract class Source {
     return nativeGetAttribution();
   }
 
+  /**
+   * Retrieve current pre-fetching zoom delta.
+   *
+   * @return current zoom delta or null if not set.
+   */
+  @Nullable
+  public Integer getPrefetchZoomDelta() {
+    return nativeGetPrefetchZoomDelta();
+  }
+
+  /**
+   * Set the tile pre-fetching zoom delta for current source. Pre-fetching makes sure that a low-resolution
+   * tile at the (current_zoom_level - delta) is rendered as soon as possible at the
+   * expense of a little bandwidth.
+   * If delta has not been set or set to null, it will use the value in MapboxMap instance.
+   *
+   * @param delta zoom delta
+   */
+  public void setPrefetchZoomDelta(@Nullable Integer delta) {
+    nativeSetPrefetchZoomDelta(delta);
+  }
+
   /**
    * Internal use
    *
@@ -85,6 +109,14 @@ public abstract class Source {
   @Keep
   protected native String nativeGetAttribution();
 
+  @NonNull
+  @Keep
+  protected native Integer nativeGetPrefetchZoomDelta();
+
+  @NonNull
+  @Keep
+  protected native void nativeSetPrefetchZoomDelta(Integer delta);
+
   public void setDetached() {
     detached = true;
   }