Merged from Docking branch: non-const ImVec2[] operator.

imgui.h

@@ -170,7 +170,8 @@ struct ImVec2
     float     x, y;
     ImVec2()  { x = y = 0.0f; }
     ImVec2(float _x, float _y) { x = _x; y = _y; }
-    float operator[] (size_t i) const { IM_ASSERT(i <= 1); return (&x)[i]; }    // We very rarely use this [] operator, the assert overhead is fine.
+    float  operator[] (size_t idx) const { IM_ASSERT(idx <= 1); return (&x)[idx]; }    // We very rarely use this [] operator, the assert overhead is fine.
+    float& operator[] (size_t idx)       { IM_ASSERT(idx <= 1); return (&x)[idx]; }    // We very rarely use this [] operator, the assert overhead is fine.
 #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