guard against concurrent OpenGL extension loading

include/mbgl/android/native_map_view.hpp

@@ -54,8 +54,6 @@ public:
 private:
     EGLConfig chooseConfig(const EGLConfig configs[], EGLint numConfigs);
 
-    void loadExtensions();
-
     bool inEmulator();
 
 private:
@@ -80,8 +78,6 @@ private:
 
     bool firstTime = false;
 
-    bool usingDepth24 = false;
-
     bool fpsEnabled = false;
     double fps = 0.0;
 };

include/mbgl/platform/default/headless_view.hpp

@@ -60,6 +60,8 @@ private:
     GLXPbuffer glxPbuffer = 0;
 #endif
 
+    bool extensionsLoaded = false;
+
     GLuint fbo = 0;
     GLuint fboDepthStencil = 0;
     GLuint fboColor = 0;