Fixed ImVec2 operator[] violating aliasing rules causing issue with Intel C++ compiler. (#6272)

Note that this is not BayesBug's exact intended solution, so issues would be my responsibility ;)
Amended.

imgui.h

@@ -255,8 +255,8 @@ struct ImVec2
     float                                   x, y;
     constexpr ImVec2()                      : x(0.0f), y(0.0f) { }
     constexpr ImVec2(float _x, float _y)    : x(_x), y(_y) { }
-    float  operator[] (size_t idx) const    { IM_ASSERT(idx == 0 || idx == 1); return (&x)[idx]; }  // We very rarely use this [] operator, the assert overhead is fine.
-    float& operator[] (size_t idx)          { IM_ASSERT(idx == 0 || idx == 1); return (&x)[idx]; }  // We very rarely use this [] operator, the assert overhead is fine.
+    float& operator[] (size_t idx)          { IM_ASSERT(idx == 0 || idx == 1); return ((float*)(char*)this)[idx]; } // We very rarely use this [] operator, so the assert overhead is fine.
+    float  operator[] (size_t idx) const    { IM_ASSERT(idx == 0 || idx == 1); return ((const float*)(const char*)this)[idx]; }
 #ifdef IM_VEC2_CLASS_EXTRA
     IM_VEC2_CLASS_EXTRA     // Define additional constructors and implicit cast operators in imconfig.h to convert back and forth between your math types and ImVec2.
 #endif