dep/imgui: Fix memory leak in imgui_freetype

Only once on font load, but still a leak.

dep/imgui/src/imgui_freetype.cpp

@@ -198,6 +198,8 @@ bool ImGui_ImplFreeType_FontSrcData::InitFont(FT_Library ft_library, ImFontConfi
           VarDesignCoords = nullptr;
           VarDesignNumAxis = 0;
       }
+
+      FT_Done_MM_Var(ft_library, mmvar);
     }
 
     // Convert to FreeType flags (NB: Bold and Oblique are processed separately)