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/card_manager.cpp

@@ -115,7 +115,6 @@ namespace overlay::windows {
             ImGui::BeginDisabled();
             ImGui::Button("  <No card>  \n  xxxx xxxx xxxx xxxx  ");
             ImGui::EndDisabled();
-            ImGui::PopStyleColor(4);
         }
     }