Prevent imgui_internal.h from being included without imgui.h (piles of errors)

imgui_internal.h

@@ -7,6 +7,10 @@
 
 #pragma once
 
+#ifndef IMGUI_VERSION
+#error Must include imgui.h before imgui_internal.h
+#endif
+
 #include <stdio.h>      // FILE*
 #include <math.h>       // sqrtf()