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/window.cpp

@@ -99,10 +99,9 @@ void overlay::Window::build() {
             for (auto &child : this->children) {
                 child->build();
             }
-
-            // end window
-            ImGui::End();
         }
+        // end window
+        ImGui::End();
 
         if (this->remove_window_padding) {
             ImGui::PopStyleVar();