[core] Ensure that context resources are destroyed before context is destroyed

include/mbgl/map/backend.hpp

@@ -41,8 +41,7 @@ protected:
     virtual void activate() = 0;
     virtual void deactivate() = 0;
 
-private:
-    const std::unique_ptr<gl::Context> context;
+    std::unique_ptr<gl::Context> context;
 
     friend class BackendScope;
 };

platform/default/mbgl/gl/headless_backend.cpp

@@ -1,5 +1,7 @@
 #include <mbgl/gl/headless_backend.hpp>
 #include <mbgl/gl/headless_display.hpp>
+#include <mbgl/gl/context.hpp>
+#include <mbgl/map/backend_scope.hpp>
 
 #include <cassert>
 #include <stdexcept>
@@ -15,6 +17,8 @@ HeadlessBackend::HeadlessBackend(std::shared_ptr<HeadlessDisplay> display_)
 }
 
 HeadlessBackend::~HeadlessBackend() {
+    BackendScope scope(*this);
+    context.reset();
 }
 
 void HeadlessBackend::activate() {