[client] imgui: use improved high DPI rendering

This actually makes imgui render at a higher resolution, avoiding scaling
and resulting blurriness.

client/include/overlay_utils.h

@@ -23,6 +23,15 @@
 
 #include "common/types.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void overlayGetImGuiRect(struct Rect * rect);
+void imGuiResetStyle(void);
+
+#ifdef __cplusplus
+}
+#endif
 
 #endif