Allow overriding the gfx backend at runtime

You can now specify -vulkan, -metal, etc to the SDL backend.

engine/utility/include/utility.hpp

@@ -111,4 +111,14 @@ namespace utility {
 
         return linear;
     }
+
+    template<typename T, typename F>
+    inline void move_to_front(std::vector<T>& vec, F func) {
+        auto result = std::find_if(vec.begin(), vec.end(), func);
+        if (result != vec.end()) {
+            auto value = *result;
+            vec.erase(result);
+            vec.insert(vec.begin(), value);
+        }
+    }
 }

platforms/sdl/main.cpp.in

@@ -407,21 +407,46 @@ void try_initialize() {
     }
 }
 
+struct gfx_backend_initializer {
+    gfx_backend_initializer(std::string_view n, std::function<void()> f) : name(n), init_func(f) {}
+
+    std::string_view name;
+    std::function<void()> init_func;
+};
+
+std::vector<gfx_backend_initializer> gfx_backend_order;
+
 int main(int argc, char* argv[]) {
     SDL_Init(SDL_INIT_VIDEO | SDL_INIT_TIMER);
 
+    engine = new prism::engine(argc, argv);
+
     // determine gfx context at the beginning
 #ifdef ENABLE_METAL
+    gfx_backend_order.emplace_back(gfx_backend_initializer{"metal", []{
         try_initialize<GFXMetal>();
+    }});
 #endif
 
 #ifdef ENABLE_VULKAN
+    gfx_backend_order.emplace_back(gfx_backend_initializer{"vulkan", []{
         try_initialize<GFXVulkan>();
+    }});
 #endif
 
+    gfx_backend_order.emplace_back(gfx_backend_initializer{"dummy", []{
         try_initialize<GFXDummy>();
+    }});
 
-    engine = new prism::engine(argc, argv);
+    for(auto arg : engine->command_line_arguments) {
+        utility::move_to_front(gfx_backend_order, [arg](gfx_backend_initializer& init) {
+            return arg == "-" + std::string(init.name);
+        });
+    }
+
+    for(auto backend : gfx_backend_order) {
+        backend.init_func();
+    }
 
     app = new @APP_CLASS@();
     engine->set_app(app);