overlay: UI scaling option (#495)

## Link to GitHub Issue, if one exists
#477 

## Description of change
Add a new option for scaling the overlay.

Gitadora Arena will receive 250% by default.

Various overlay windows have been updated to scale properly, not perfect
but they are at least usable.

src/spice2x/overlay/overlay.h

@@ -3,6 +3,7 @@
 #include <memory>
 #include <mutex>
 #include <functional>
+#include <optional>
 #include <vector>
 #include <windows.h>
 #include <d3d9.h>
@@ -27,6 +28,18 @@ namespace overlay {
     extern bool USE_WM_CHAR_FOR_IMGUI_CHAR_INPUT;
     extern bool FPS_SHOULD_FLIP;
     extern bool SHOW_DEBUG_LOG_WINDOW;
+    extern std::optional<uint32_t> UI_SCALE_PERCENT;
+
+    template <typename T>
+    float apply_scaling(T input) {
+        if (!UI_SCALE_PERCENT.has_value()) {
+            return input;
+        }
+        return static_cast<float>(input) * UI_SCALE_PERCENT.value() / 100.f;
+    }
+
+    ImVec2 apply_scaling_to_vector(const ImVec2& input);
+    ImVec2 apply_scaling_to_vector(float x, float y);
 
     class SpiceOverlay {
     public: