example: gurantee destroying user data before canvas destruction

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Pull request overview

This PR adjusts teardown order in the example window backends so user Example state is destroyed before the ThorVG Canvas, and moves Example/Canvas deletion out of the base Window destructor into backend-specific destructors.

Changes:

• Remove example/canvas deletion from Window::~Window().
• Add SwWindow destructor to delete example and canvas.
• Update GlWindow/WgWindow destructors to delete example before canvas (and remove canvas = nullptr).

Comments suppressed due to low confidence (2)

src/Example.h:180

• Window::~Window() no longer deletes example/canvas, so any Window subclass that doesn’t implement a destructor to delete them will now leak (e.g., the non-TVGEXAMPLE_WGPU_SUPPORTED WgWindow variant has no destructor). Either restore ownership in Window (with correct ordering relative to backend teardown) or add destructors for every subclass/variant to delete example (and canvas when applicable).

    virtual ~Window()
    {
        //Terminate the SDL
        SDL_DestroyWindow(window);
        SDL_Quit();

        //Terminate ThorVG Engine
        tvg::Initializer::term();
    }

src/Example.h:513

• WgWindow can early-return from its constructor when !initialized, leaving instance/surface/adapter/device uninitialized. The destructor unconditionally releases all of them, which is undefined behavior if creation didn’t complete. Initialize these handles to nullptr at declaration and guard each wgpu*Release() so it only runs for non-null handles.

    virtual ~WgWindow()
    {
        delete(example);
        delete(canvas);

        wgpuDeviceRelease(device);
        wgpuAdapterRelease(adapter);
        wgpuSurfaceRelease(surface);
        wgpuInstanceRelease(instance);
    }

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