[core] Create BackendScopes in Renderer, assert in Impl

This avoids a long series of reset() calls in the Impl.

src/mbgl/renderer/renderer.cpp

@@ -15,7 +15,10 @@ Renderer::Renderer(RendererBackend& backend,
                                       contextMode_, std::move(programCacheDir_))) {
 }
 
-Renderer::~Renderer() = default;
+Renderer::~Renderer() {
+    BackendScope guard { impl->backend };
+    impl.reset();
+}
 
 void Renderer::setObserver(RendererObserver* observer) {
     impl->setObserver(observer);
@@ -72,6 +75,7 @@ void Renderer::dumpDebugLogs() {
 }
 
 void Renderer::onLowMemory() {
+    BackendScope guard { impl->backend };
     impl->onLowMemory();
 }
 

src/mbgl/renderer/renderer_impl.cpp

@@ -64,13 +64,7 @@ Renderer::Impl::Impl(RendererBackend& backend_,
 }
 
 Renderer::Impl::~Impl() {
-    BackendScope guard { backend };
-    glyphManager.reset();
-    imageManager.reset();
-    lineAtlas.reset();
-    renderSources.reset();
-    renderLayers.reset();
-    staticData.reset();
+    assert(BackendScope::exists());
 };
 
 void Renderer::Impl::setObserver(RendererObserver* observer_) {
@@ -670,7 +664,7 @@ std::vector<Feature> Renderer::Impl::querySourceFeatures(const std::string& sour
 }
 
 void Renderer::Impl::onLowMemory() {
-    BackendScope guard { backend };
+    assert(BackendScope::exists());
     backend.getContext().performCleanup();
     for (const auto& entry : renderSources) {
         entry.second->onLowMemory();