ImGuiManager: Split OSD and debug window fonts

Fixes the latter not fitting on screen anymore.

src/util/imgui_manager.h

@@ -69,6 +69,9 @@ bool AddFullscreenFontsIfMissing();
 /// Returns the standard font for external drawing.
 ImFont* GetStandardFont();
 
+/// Returns the standard font for on-screen display drawing.
+ImFont* GetOSDFont();
+
 /// Returns the fixed-width font for external drawing.
 ImFont* GetFixedFont();