[core] fix backend scope handling on render, cleanup and lowmemory

- Make renderer frontend responsible for creating a BackendScope on render
- encapsulate backend scope handling on cleanup and low memory in renderer impl

include/mbgl/renderer/renderer_backend.hpp

@@ -30,10 +30,6 @@ public:
     // Called prior to rendering to update the internally assumed OpenGL state.
     virtual void updateAssumedState() = 0;
 
-    virtual BackendScope::ScopeType getScopeType() const {
-        return BackendScope::ScopeType::Explicit;
-    }
-
 protected:
     // Called with the name of an OpenGL extension that should be loaded. RendererBackend implementations
     // must call the API-specific version that obtains the function pointer for this function,

src/mbgl/renderer/renderer_impl.cpp

@@ -35,7 +35,7 @@ Renderer::Impl::Impl(RendererBackend& backend_,
 }
 
 Renderer::Impl::~Impl() {
-    BackendScope guard { backend, backend.getScopeType()};
+    BackendScope guard { backend };
     renderStyle.reset();
     staticData.reset();
 };
@@ -47,8 +47,8 @@ void Renderer::Impl::setObserver(RendererObserver* observer_) {
 void Renderer::Impl::render(View& view, const UpdateParameters& updateParameters) {
     // Don't load/render anyting in still mode until explicitly requested.
     if (updateParameters.mode == MapMode::Still && !updateParameters.stillImageRequest) return;
-
-    BackendScope guard { backend, backend.getScopeType() };
+    
+    assert(BackendScope::exists());
 
     renderStyle->update(updateParameters);
     transformState = updateParameters.transformState;
@@ -348,7 +348,7 @@ void Renderer::Impl::onResourceError(std::exception_ptr ptr) {
 }
 
 void Renderer::Impl::onLowMemory() {
-    BackendScope guard { backend, backend.getScopeType() };
+    BackendScope guard { backend };
     backend.getContext().performCleanup();
     renderStyle->onLowMemory();
     observer->onInvalidate();