[core] refactor RendererBackend

platform/android/src/map_renderer.cpp

@@ -1,6 +1,7 @@
 #include "map_renderer.hpp"
 
 #include <mbgl/renderer/renderer.hpp>
+#include <mbgl/gfx/backend_scope.hpp>
 #include <mbgl/util/shared_thread_pool.hpp>
 #include <mbgl/util/run_loop.hpp>
 
@@ -159,6 +160,9 @@ void MapRenderer::onSurfaceCreated(JNIEnv&) {
     // Lock as the initialization can come from the main thread or the GL thread first
     std::lock_guard<std::mutex> lock(initialisationMutex);
 
+    // The GL context is already active if get a new surface.
+    gfx::BackendScope backendGuard { *backend, gfx::BackendScope::ScopeType::Implicit };
+
     // The android system will have already destroyed the underlying
     // GL resources if this is not the first initialization and an
     // attempt to clean them up will fail