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/overlay.h

@@ -26,6 +26,7 @@ namespace overlay {
     extern bool AUTO_SHOW_KEYPAD_P2;
     extern bool USE_WM_CHAR_FOR_IMGUI_CHAR_INPUT;
     extern bool FPS_SHOULD_FLIP;
+    extern bool SHOW_DEBUG_LOG_WINDOW;
 
     class SpiceOverlay {
     public:
@@ -128,6 +129,7 @@ namespace overlay {
         bool hotkey_toggle_last = false;
 
         void init();
+        void add_font(const char* font, ImFontConfig* config, const ImWchar* glyphs);
     };
 
     // global