Expose backend event handling features

[Demorome]

This is mostly so anyone can easily use the ImGui SDL_GPU backend's ProcessEvents to automatically handle all SDL events.

For example, my OnReceiveEvent callback currently looks like:

    private unsafe void ProcessEvents(SDL.SDL_Event e)
    {
        ImGuiImplSDL3.ProcessEvent((ImSDLEvent*)&e);

        // Prevent editor-captured inputs from leaking over into the game.
        var io = ImGui.GetIO();
        Game.PreventKeyboardInputs = io.WantCaptureKeyboard;
        Game.PreventMouseInputs = io.WantCaptureMouse;
    }

(Using the Hexa ImGui C# port for the ImGui SDL_GPU backend).

[Demorome]

Marking this as a draft, since I found an issue with my implementation for the Prevent(...)Inputs option:

If an input was pressed and PreventInputs is switched on, then later switched off, it might discard the input-released event and leave the input in a pressed-down state, at least until the button is re-pressed and let go.

[Demorome]

The fix I went with was to just constantly set buttons as non-pressed and deltas to 0 while inputs are prevented.

I could do a cached bool check to prevent redundant changes but that didn't seem necessary.