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/external/imgui/backends/imgui_impl_dx9.cpp

@@ -327,7 +327,7 @@ void ImGui_ImplDX9_InvalidateDeviceObjects() {
     if (g_FontTexture) {
         g_FontTexture->Release();
         g_FontTexture = NULL;
-        ImGui::GetIO().Fonts->TexID = NULL;
+        ImGui::GetIO().Fonts->TexID = 0;
     } // We copied g_pFontTextureView to io.Fonts->TexID so let's clear that as well.
 }