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/windows/control.cpp

@@ -178,9 +178,11 @@ namespace overlay::windows {
                     static_cast<int>(ImGui::GetIO().DisplaySize.x),
                     static_cast<int>(ImGui::GetIO().DisplaySize.y));
 
-            // removed for size (along with setting IMGUI_DISABLE_DEMO_WINDOWS
-            // and IMGUI_DISABLE_DEBUG_TOOLS) - saves about 300kb in each
-            // binary
+            if (ImGui::Button("ImGui Debug Log")) {
+                overlay::SHOW_DEBUG_LOG_WINDOW = !overlay::SHOW_DEBUG_LOG_WINDOW;
+            }
+
+            // removed for size (IMGUI_DISABLE_DEMO_WINDOWS)
 
             // metrics button
             // this->metrics_open |= ImGui::Button("Metrics Window");