Refactor Renderer access and enhance graphics loading functionality

- Replaced instances of Renderer::GetInstance() with Renderer::Get() for consistency across the codebase.
- Updated various rendering methods to streamline bitmap updates and enhance performance.
- Removed unused includes and comments to improve code clarity and maintainability.

src/app/core/controller.h

@@ -7,13 +7,7 @@
 
 #include "absl/status/status.h"
 #include "app/core/platform/backend.h"
-#include "app/core/platform/file_dialog.h"
-#include "app/core/platform/renderer.h"
 #include "app/editor/editor_manager.h"
-#include "imgui/backends/imgui_impl_sdl2.h"
-#include "imgui/backends/imgui_impl_sdlrenderer2.h"
-#include "imgui/imconfig.h"
-#include "imgui/imgui.h"
 
 int main(int argc, char **argv);