imgui: update to v1.91.4, make various fixes to overlay (#286)

## Link to GitHub Issue, if one exists
n/a

## Description of change
Update ImGui to `v1.91.4`. We can't update to `v1.91.5` or newer just
yet because impl_spice depends on the old IO model of ImGui which got
removed in `v1.91.5`.

We also can't update the DX9 renderer right now, because at some point
overlay breaks in TDJ (and only in TDJ...)

Enable the new debug logging functionality in ImGui. Add a button in
overlay to toggle debug log window.

Fix some minor bugs spotted by the new ImGui debug log: mismatched API
calls, font loading issues.

## Compiling
👍 

## Testing
In progress.

src/spice2x/overlay/imgui/impl_sw.cpp

@@ -678,7 +678,7 @@ void bind_imgui_painting()
 	int font_width, font_height;
 	io.Fonts->GetTexDataAsAlpha8(&tex_data, &font_width, &font_height);
 	const auto texture = new Texture{tex_data, font_width, font_height};
-	io.Fonts->TexID = texture;
+	io.Fonts->TexID = reinterpret_cast<ImTextureID>(texture);
 }
 
 static Stats s_stats; // TODO: pass as an argument?